I would see it as javascript's strict mode, and I think most devs would ok with a binary switch.

The only fear I'd have is a lack of big entity committed to PHP. In the heydays there was Yahoo, facebook and other giant companies relying on PHP and guaranteeing that there would be enough man power to weather any type of changes. Now I wonder how many people are left to push through breaking changes and rewrite/modernize the libraries and the ecosystem.

PHP already has a strict mode as well. It's pretty hard to imagine every developer typing in a long block of compatibility switches at the top of every file and then the mental load of maintaining a code base with some of those on and off.

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.

That's why you want to do it on a per file basis, not on a per interpreter basis, which allows a project to incrementally move.

You also want a single version, not a gigantic pile of things to opt in to.

The complexity still has to exist somewhere, right? By avoiding this, they're just keeping it in all the developers' heads, rather than a legacy mode in the compiler.

I'm not sure that's the wrong decision, but it's not a great one, either. Too much complexity in the implementation can kill the project. Too much complexity in the language can kill a person's hope of learning it.

Many complaints are about the PHP standard libraries and not about the language itself, so you can learn the language just fine and just don't learn the old deprecated functions at all unless you hit such a function in a codebase and tehn you research it. . Is not equivalent to compare PHP+ standard library with JS core , PHP included function to read and write files, streams, databases,json,XML, run executables(if enabled), where in JS or other languages most of this functions are not standard.

The libraries can also be treated in a similar manner.