When a deprecation is thrown inside your very src/, you want it to always make your CI fail, don't you?
I propose to update the behavior of the SYMFONY_DEPRECATIONS_HELPER=123456 mode to make it mean:

make the CI fail if vendors throw more than 123456 deprecations (and make it always fail if any deprecations are thrown by my src/.)

This would be a more powerful alternative to weak_vendor , which could be deprecated.

WDYT?

(#27006 could converge with this RFC I suppose.)