There's already been a degree of that with php.ini flags, and some of the behaviours available via those flags have been deprecated then removed over a period o

There's already been a degree of that with php.ini flags, and some of the behaviours available via those flags have been deprecated then removed over a period of years. It's not impossible. Yes, it's complex, but the process for dealing with that complexity has already existed and worked out ... relatively smoothly. The bigger issue is getting people to upgrade - I'm still dealing with a couple of projects - one started in 2017 - that are on PHP 5.6.