Can an app use MPTCP if they don't have a MPTCP-aware server component? Or is Apple proxying (or via VPN) the MPTCP connection as part of their services?
It needs server side support, but the OS just supports it out of the box. On linux enabling multi path is I believe just a configuration flag and then it just works.
referencing : https://support.apple.com/en-sg/101905
and the other comments on lack of MPTCP support from middleboxes, my guess is even for Apple first party apps, out in the wild, might not see much MPTCP use?