Mutation Events were once in all 3 moderns engines plus some legacy ones and is now removed from everything.

Third-party cookies are now heavily restricted and while not quite removed everywhere cannot be relied upon.

document.domain is no longer settable by default in Chromium, and other browsers are aligned on matching this.

There's other APIs that have been restricted to secure contexts only.

The list goes on and on, the idea that the web is this unbreakable surface isn't true. It does break things and that is a good thing, if you want the platform to succeed for decades to come they should be able to fix mistakes.