Odd, I tried to go back to Act 1 from Act 2 (looking for a vendor since spoiler stuff happened at the only settlement in cursed lands) and when I clicked the elevator it told me I “really shouldn’t” like twice and the third time it game over’d me in a unique and creative way… I felt that was a pretty hard lock out of act 1… Lol
I think there is a grace period after hitting act 2 where you still can go back but once you’ve done a bit of story related stuff you are pretty locked in.