[meta] Mark 'half-converted' classes somehow

Is this still the case?

Suspect there has been some work in this space over the last few years. Can this be closed now?

Otherwise sounds like the next steps here are to identify the relevant classes and add an appropriate comment.

It's more relevant than ever because core just moved old procedural code into Hooks classes.

Almost all of Views code is unchanged since 7.x-3.x or even older.

And so on.