I did some more checking and according to IETF, the UIDONLY spec is still an internet-draft and has not been formally added to the IMAP standard. It should not be implemented for public use, which would certainly explain why Outlook and other IMAP clients don't appear to support it.
In fact, the UIDONLY spec contains this wording:
"IMPLEMENTATION NOTE: this document is not yet at the state where it is implementable. Please contact document authors if you want to experiment with implementing UIDONLY."
Interestingly, there's nothing about UIDONLY that specifically relates to how many messages a client can see. I can't understand why client support for this would be needed to access more than 10,000 messages.
(For background, UID in IMAP means "unique ID" and it is a number assigned to a message which doesn't change as long as the message itself doesn't change. UIDs have been in IMAP for years and simplify client implementation. Besides UIDs, messages also have a "message number" which is ephemeral. UIDONLY tells the server that the client doesn't need or use the ephemeral message numbers. Why UIDONLY is now needed by Yahoo/AOL IMAP clients to see the entirety of messages in a folder is a mystery and not required or mentioned by the spec.)
And for what it's worth, 3 of the 6 authors of the UIDONLY draft are from Yahoo.
Here's a link to the spec draft:
https://www.ietf.org/archive/id/draft-melnikov-imap-uidonly-00.html
And here's the IETF status page for it:
https://datatracker.ietf.org/doc/draft-melnikov-imap-uidonly/