I'm slightly unsure why we need this. Yes, it "simplifies" the class definition, but on the other hand it also removes some self-documenting aspects of it.

Do you have other reasons for this beside "we can do without"?

The main reason I'd say is that it is a cause of confusion when learning how to build ggplot2 extensions because it isn't really clear what should go intooptional_aes. It is better now that#6426 describes the field, but it remains somewhat redundant and befuddles a clear path for how to implement optional aesthetics.