Thank you @davidromani.

The issue when putting this before the class declaration is that the notice is triggered only for the place triggering the autoload. So it will report a deprecation only in 1 place, not in all places using it

Which is good, because we do not want people to get flooded, but we only want to raise their awareness.
Then they can grep.

@nicolas-grekas the log message includes the location of the deprecated usage. Here, we are making things harder

I don't see how this is different with putting the notice inside the method/constructor?
Maybe be should adjust the logger for deprecation notices so that the n-1 frame in the stack is reported?

@nicolas-grekas in the constructor, it is reported for all places instantiating it. With the deprecation near the declaration, it is triggered when the autoloader includes the file, which gives you only the first usage.
Grepping to find similar usages might be complex because of dynamic access