League of Legends (LoL) Question: Is jayce still viable adc?

He was never a viable AD carry. Right now he's usually played top lane (or occasionally mid versus other AD mids) with a primarily AD-centric build. You'll usually see items like Bloodthirster, Black Cleaver, Last Whisper, Ionian Boots of Lucidity (or defensive boots) and Muramana.

I still think he's pretty good in high-mobility comps or poke-heavy comps since his E-Q in ranged form hits pretty hard with an offensive build. You can still switch to hammer form when the fight starts and peel for your carries or clean up if your team is winning.

That's as close as you're probably going to get to playing him as an ADC though, at least in terms of viability. Even with a ton of AD, he isn't an AD carry. He simply can't use crit and attack speed effectively with his attack range and abilities. AD carries need more than just damage, they need DPS, which is something that you can't really afford to build on Jayce.

He was never an amazing carry for bot lane due to his low attack range (500). However, his bruiser part made up for a lot. He got a bunch of nerfs and I'm pretty sure he isn't good anymore there.

I'd advise you to play something else.

No. He was attempted as an ADC but in the end it doesn't work because he doesn't supply what an ADC needs.
He's good in a dual-bruiser bot though I think

He can go bot lane if a carry goes elsewhere, like top Graves or something.

It's a lane swap thing, Jayce makes a kill lane, done it before with a Leona support. Jarvan IV does it better though.

I don't get it in the champion screen Riot has him under the rest of the adc's (marksmen) so i guess they think he is viable ... I don't agree 100% with them, but will try, who knows maybe he is beter now.

When was he ever?