If I'm not wrong, in userland PHPStan will report that classes implementing NormalizerInterface and DenormalizerInterface will have to specify @implements with correct types.

Is it considered as a BC or its fine, since it only "break" analysis tools and not the code at runtime itself?

If I'm not wrong, in userland PHPStan will report that classes implementing NormalizerInterface and DenormalizerInterface will have to specify @implements with correct types.

Yes, but this is a classic behavior when using libraries with static analysis, bumping patch/minor might introduce new warning ; also Phpstan offers a config option to ignore some classes for such errors.

Is it considered as a BC or its fine, since it only "break" analysis tools and not the code at runtime itself?

This is considered as backward compatible, and such changes has already been merged in minor version (the Voter Pr is an example).