Jayce is currently sitting on the PBE with Base HP, HP Regen, and their scaling all getting nerfed. Now, I do think that Jayce needs a nerf. He's at about 49% winrate, which for Jayce is pretty high considering his ceiling, that being said the HP nerfs make no sense.
The annoying part of Jayce is his abusive laning phase thanks to his high damage from range. The reason the nerf is going through in the first place is to "bring him in line with other marksmen" since that's where "he's currently at." I don't think we should be moving him more towards a pure marksman though if that's the problem, instead his power should be shifted over to hammer stance. If the issue is his abusive laning phase, nerf his cannon damage and mitigate that with some changes to his hammer damage so he feels forced to go out of his comfort zone and take risks that the opposing laner can actually capitalize on.
tl;dr The main issues with this nerf are that 1. these changes completely miss the mark and won't help make facing Jayce in lane any more bearable at early levels, and 2. it's purposefully changing Jayce's identity as a champion, something that should be avoided just for the sake of balance (specifically when there are alternatives).