I guess I must edit a CHANGELOG.md but I don't understand how it work.

Is it src/Symfony/Contracts/CHANGELOG.md? what version should I pick?

Tests fail, we need to override some methods from the base class.

Should we do this only when the contracts are used? Aka when using the PSR-6 interface, still throw?

I've described why it would be better to still forbid semicolons in cache keys in the #45599.

I'm closing here because this was a poc and it showed we need more discussion - also because the implementation is likely not the one we'll want.