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Javascript is a client-side language, you don't have control of the software stack, thus (ideally, if you want to) you have to write code to support every version, even a prehistoric chrome build from 2015 (be it directly or indirectly via transpiling and polyfills).

PHP is not like that, like in every other language where you can control the entire environment, it evolves and things break, and that's the way it should be, that's why we have semantic versioning.

If you are upgrading your web software stack, why aren't you upgrading your web software?

There are services you can pay for that backport security fixes to old versions of php.