I'm kind of surprised they'd change a core language semantic, even in a major update... won't this make upgrading to PHP8 for a large codebase hard to do with confidence?
That was certainly a concern. When instrumenting PHP to detect where the behavior changes and testing various codebases, I found that this actually happens much more rarely than I would have anticipated in advance. We're talking like 2 occurrences in the test suite of major frameworks.
Strict types (scalar type hints for method signatures) have been around since PHP 7, and since then I would say it is recommended to always use === (with type check) over == (no type check) so one would have enough time to upgrade it‘s code.
And it also just makes sense from the language design perspective to fix that behaviour. Having a major version coming is a good time to fix such stuff imho.