I have a couple lists on a SPO site that do not get the "Forms Settings" option under List Settings. It is under Integrate, and I have successfully created new PowerApp forms for a couple lists and they work fine. Although, since I do not appear to have that specific setting, I cannot manage it from the list level (Like reverting back to default).
Every other list has this option in it, except all the lists on this site.
This site is a private channel from a Team, as that seems likely why. Although I have no idea why it would let me customize the forms, yet not even show me that menu option in actual List Settings.
I CAN get to it if I add FormSettings.aspx? to the URL.