You do make a good point.
Well they updated BFT about two weeks before S4 preseason, so older guides do not actually cover this. Technically it is not banned, and I would endorse not using any at all, but if that circumstance does arise, I would say perhaps building against your champions' scale.
For example if you are playing twitch, then go ahead because BFT does not scale with anything you own. But for champions like leblanc perhaps be a bit more considerate and build zephyrs, because in the end, the actions of hiders reflect how long seekers are going to be willing to play.
For a list of unbanned AP items, you should look into items that give movement speed or cooldowns and/or (if you are building the slow item or oracles) hextech gunblade from bligewater cutlass and rylais are AP movespeed slowing items. Hexsweeper is the AP oracles item.
Keep in mind that the cap on cool-down is 40%, so once you build 40%, any more cdr will be build in vain. So if we consider that you build one zephyr (for everything plus tenacity) - that is 10%, if you perhaps build a talisman of ascension which gives 20% cdr and then thats already 30% and the other 10 can come from another zephyr, a hexsweeper, or even a BFT; so there is no need to go into building large AP items for cdr (morellonomicon or example).

Thannks from answer. It may be case, that 200 gold doesn't matter in Hide and Seek, but you said also, that it doesn't matter in dominion. It matters in dominion, since lasts only 7-18min, 200 gold is big portion of all money earnt. BFT also builds from Fiendish codex, that costs 430 less than Stinger. When I want go fast in dominion, my build order is boots of mobility, bft, two fiendish codexes, two bft, I never get that long, but if for some reason game still continues, I start building zeals. Is bft banned for hiders? Can you give list of ap items, that are unbanned for hiders?

Great question.
And the attackspeed, attack damage is precisely the reason why multiple zephyrs are better, and this is because hiders are not supposed to auto-attack, therefore that is one form of damage taken away from hiders. If a hider instead builds multiple blackfires, then every cc skill (given it scales off AP where most chosen hiders do) they use will do more damage to seekers and send them recalling before they have the chance to actually "seek" if you can relate.
Zephyr would therefore only be there to give the 10% MS adjustment and 10% cool down which is all hiders really need. If you give hiders 80AP along with that and let them stack it up, imagine the deterrence it would be for seekers. You might have also noticed that there is a significantly lower ratio of seekers to hiders and this is due to many factors- courtesy being one and hiders that troll being the other.
And in relation to the 200 gold factor, it does not make that great of a difference in The Crystal Scar/Dominion if you have noticed and it is due to the consistent amount of gold rewarded at all times, more than often you will find that you can either purchase another item like talisman of ascension or youmuu's or even quicksilver sash instead due to the lack of gold you have in general/or left over.

Why do you consider for HIders to build multiple Zephyrs? One zephyr sounds legit, because tenacity, but then BFT does same thing expct it costs 200 less. Is that ad and attackspeed really so relevant for hiders?

Witchcap is in no way a defensive item in my opinion, but you can definitely argue it otherwise. And my reasoning for this is because it is an item which delays inevitable death, similar to guardian angel but obviously in a different way. I assume you have accepted that Guardian Angel is an item that hiders have never been allowed to purchase, but the witchcap was introduced in 2013 after the remake of zhonya's so past guides have not been able to cover this item.
The guardian angel brings a hider back from death with additional fixed hp and mana but also renders the user invulnerable to attack until revival, I consider this the same as the stati affect from witchcap (an item which is otherwise just pure AP), witchcap is most commonly used at the brink of death to preserve the last shread of hp left on a person, but that is near equivalent to death, and if you wanted to argue the witchcap's defensive side keeping in mind that the item itself provides the most amount of flat AP damage on any item that can be bought on dominion, it would be interesting; because you could alternatively argue the defensive stats on the Guardian Angel which don't provide any source of damage.
There are alternative items that can be bought to provide the HP of RoA's final stage without the addition of its AP.

In terms of no oracles till 15. That rule has been there since the earliest anyone has ever played HNS and also in the earliest guides I've read, some actually I can't find any more because the sites have closed or changed. But there are numerous games which I have played first hand where the stealth advantage and no oracles actually does not render those characters impossible to kill, and on the contrary, more and more games as of recently hone more skilled seekers who are capable of finishing a game in 25 minutes and under regardless of whether there are champions that can use stealth or not.
Alternatively there are also 3 bans that can be made and in a majority of games, 3/4 of those champions are banned and the other one is "pre-banned" by the hosts. So unless the seekers have their hands tied behind their backs, if stealth champions are an issue, there is always the option to draft ban those champions.
But those are definitely good points you have brought up, and every game varies with the hosts and what rules/guide they play by.
I would appreciate if you reconsidered the witchcap as more of an offensive item than a defensive item, and that also shares similarities with the guardian angel, which has never been allowed in hide and seek games. In addition, the hiders should also not build for items that do more damage on top of abilities which already use base damage + cc effect on seekers.

No Witchcap/Rod of Ages? Really? They're both arguably defensive items.

No oracle's until 15 is the most ******ed rule ever. This just automatically makes champions like Teemo, Evelynn, Twitch, Shaco etc. nearly impossible to successfully kill 5 times because they effectively only have to hide for 25 minutes, as opposed to 40.

It does count, and I'll put this in the revised F.A.Q section too.
From my experience playing in 2012-early 2013 before a thousand other HNS guides popped up, a seeker is permitted to chase and kill a hider even below 200 hp, so after giving the seeker a chance to recall (3 seconds or so), the hiders are allowed to cc the seeker to death because the seeker has decided to take a gamble in chasing.
If the seeker gets a kill after they continue chasing they should be wary that the hiders allies can still kill them so they should recall asap.
But I am aware of an interesting discussion on this particular point and that is the 3 seconds hiders give to seekers to recall, in that time the seeker is basically invulnerable and has every intention to commit to the kill, so the discussion revolves around whether or not the seeker should compensate with an additional 3 seconds for nearby hiders to kill the seeker (given that he got the kill). As opposed to getting the kill and instantly recall where the seeker is invulnerable to death even after taking that gamble.
The rules state that you need to let a seeker recall once he starts his animation so that tells us that seekers are able to commit to the kill and recall with little to no fear of dying due to the 3s of invulnerability (this is actually time given to the seeker for them to realise their hp and recall), and if we are not questioning the fairness in this, we should also keep in mind that hiders have the tendency to reward themselves with more time than stated; by saying this I am referring to if hiders get extra time to kill the seeker (if the seeker took the risk), and that would unbalance the game as well.
By saying that I mean if this rule was implemented in custom games - hiders would more than likely take/reward themselves longer than 3 seconds to kill the seeker.

Fair enough, also when seekers kill you below 200 does it count? Because there's many that say 'you can kill him if he doesn't back and keeps chasing and if u get killed it counts' or killing or assisting below 200 doesn't count.

That would be a rule for a custom Hide and Seek game. The general rules which applies to everyone is 2:00 because you already have 40 seconds to leave spawn and hide before the seekers get to leave at 2:00. But if you are still in spawn at 2:00 then theoretically you are cheating, as you are not in a position where the seekers can find and kill you at the starting time of their seeking period.
You should choose between getting philosophers stone and tier 1 boots, or tier 3 boots and no stone because its not fair for everyone else, including your team mates who more than likely play by the original and general rules.

Really nice guide, what about leaving spawn latest at 2:30 but u can get camped if doing so?

Coz I often leave spawn with philo + swiftness to earn up gold again around 2:15-2:20

I'll get onto that, I am asking people that I know if they can record.