Does it hurt? What if someone extended this? That would break BC for no obvious reasons. I think we can keep it the current way.

Well, according to the history the logger was never used in the class. So it was an error in the first place.
Otherwise we could inject the logger in every class because it could potentially be useful.

That's not what I'm saying. If someone uses it in a child class, this is a BC break.

and it is also a BC break if someone instantiate the class as it changes the signature of the constructor

Yes too bad this unused property made it into the core. Instead, a potential subclass should have overwritten the service and injected the logger himself if its needed.

How about deprecating this for 2.2? So we could remove it from 2.3.

Deprecating something leads to a BC break at some point. We already do too many BC breaks if you ask me, so let's really break BC when it really make sense.

ok