There are two RFCs out there that are in one way or another current.
I can confirm that in the latest IMAP RFC I can find, RFC 9051, FETCH RFC822 is gone. But that’s actually IMAPrev2, not IMAPrev1 - and iCloud’s IMAP services says “IMAPrev1”, which means you should be able to expect it to meet RFC 3501 - and that has FETCH RFC822.
Nor is New mentioned as a standard flag; that would be \Recent, for my purposes.
In IMAPrev2 that is “deprecated”, no explanation but it is somewhat complicated. There’s a lot more about flags and keywords that I’ve never cared to look into, that a lot of clients make extensive use of. You should be able to see the basic setup in the SELECT response.
If they have an IMAPrev2-only implementation, I don’t think they can legitimately declare IMAPrev1 - that would be multiple violations.