Why is Admin Libraries obsolete?

Hello!
I'm curious why you have marked Admin Libraries as obsolete. The code seems to work. Is there an issue that isn't obvious on first glance?

I find the need to add some assets for admin themes to be fairly common, so I think there's a valid use case here. Also, I'm considering doing something similar for a module I maintain (e.g. discovering and loading asset libraries declared by other modules or themes), so I'm curious if there's a flaw with this approach.

Thanks in advance!