This may be an issue for deprecations triggered in other libraries than Symfony though, if they don't use the new silencing pattern

Yes and no: if the deprecations where in place before the bridge was released, then these libs already have a way to make their users handle them. If the deprecations where added by taking inspiration from Symfony, then this is a recent move and I'd advocate we need to push them one step forward.
It took us several months to create the current deprecation policy and I now really like it. This bridge should help spread the word :)

Well, pushing to the new way is indeed a good idea. 👍

You should also updatehttp://symfony.com/doc/current/contributing/code/conventions.html#deprecations with the new convention btw

Doc PR issymfony/symfony-docs#5413

Thank you@nicolas-grekas.