This will not log deprecations at all when not using MonologBundle and also won't log them at all when using MonologBundle without explicitly creating a deprecation channel.

This re-introduces the same issue than what was fixed previously. The "bug" you tried to fix is that someone was relying on the previous regression as a feature rather than a bug.

So -1 for me.

To me this will log deprecations on the php channel, as wanted

I think you missed that deprecations will go on the logger defined as $logger argument when $deprecationLogger is null.

If this is indeed the case, why did we have to replace the usage of service('monolog.logger.deprecation')->nullOnInvalid() by a dynamic configuration in the compiler pass in the previous PR ?

We did that to wire "logger" by default so that when monolog is not there, we still have a logger for the php notices/deprecations.

but you now wire null by default, not logger.

Sure, that's the code in DebugHandlersListener that does the trick: