The disksummary version of POP3 has some pretty major differences.
All folders on the disksummary must be complete implementations, they must all support a summary and search interface, etc.
So, POP3 now does this. Which means the code itself now implements keep-on-server by interpreting the DELETE flag appropriately. No need for the messy UIDCache stuff and complex client code. It can just iterate all messages.
I think all pop messages are cached locally as well, but the guts of the implementation is still basically the same.
Just because it supports a summary interface, it doesn't mean the summary is complete - it only contains the UID, so searching wont actually work either.
It should be possible to remove most of the special-case POP code in the mailer and just use 'inbox filtering' to implement incoming pop mail. Rather than retrieving and filtering messages under control of the client. I'm not sure how reliable this would work in practice however.