People underestimate how often MPTCP is actually used. Siri has been using it for ages and has since expanded their MPTCP usage to many other built in apps. At some point MPTCP became available to all apps on iOS. Any network with an iPhone connected to it is using MPTCP, whether you know it or not.

When you're using TCP, you can enable MPTCP for free and make your connections faster and more stable. If you're not using TCP, there are alternatives, but then MPTCP is completely irrelevant anyway. You can use QUIC if you want to bypass shitty middleboxes, for instance, as that has similar features but smuggles itself past shitty middleboxes by being marked as UDP (which also makes it more likely to get dropped when the network is congested, unfortunately).

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?