I can't find any information on X-AcceptDeltaUpdates

IIRC, MIME (used both for Mail and for HTTP) allows to invent non-standard "experimental" headers by prefixing them with "X-". The other end should silently ignore all unknown experimental headers.

Alexander

IIRC, MIME (used both for Mail and for HTTP) allows to invent non-standard "experimental" headers by prefixing them with "X-".

Correct, though its use is now discouraged - from RFC7231 (emphasis mine):

Authors of specifications defining new fields are advised to keep the name as short as practical and not to prefix the name with "X-" unless the header field will never be used on the Internet. (The "X-" prefix idiom has been extensively misused in practice; it was intended to only be used as a mechanism for avoiding name collisions inside proprietary software or intranet processing, since the prefix would ensure that private names never collide with a newly registered Internet name; see [BCP178] for further information).

And the linked RFC6648 is dedicated to discussing the deprecation, with some interesting background in the appendicies.

Yes it appears HTTP servers should do case insensitive matches to headers by specification.

However it sort of appears by convention that HTTP libraries should not change case of headers that are specified to them.

Lesson learned: Never trust either end!