the usage of get_debug_type in FatalThrowableError also seems to cause issues (see the Goutte issue referencing that ticket)

Out of curiosity: What's the problem with using the polyfill here?

@derrabus polyfills rely on autoloading (for the internal class). So this breaks if the polyfill is called inside the autoloading stack (as autoloading the polyfill internal classes will also try to use them then).
For the error handling part, I suspect it is similar (for cases where an error happens during the error handling or during the autoloading of the polyfill internals)

I dont see us using any PHP8-only code in the ErrorHandler. I agree with @Tobion

With that change, Im 👍

Polyfill now removed (we were using a few calls to get_debug_type(), now removed).

I guess we could revert this in 6.0 as it will require php 8?

@Tobion yes (but without re-adding the polyfill package)

I did revert while merging up.