More and more apps (mostly server apps) have a dedicated option to enable MPTCP. Some server apps have even decided to enable MPTCP support by default, which makes sense: if MPTCP is not requested, TCP is used like before. Note that server apps written in Go usually have MPTCP enabled by default (if supported by the OS/kernel). See: https://www.mptcp.dev/apps.html
mptcp.io monitors servers supporting MPTCP.
> I dont think it’s well supported as a client yet
It is: by default, NetworkManager will configure MPTCP endpoints, so app can use multiple interfaces (if any). See: https://www.mptcp.dev/pm.html
> who knows if Android will ever
Sadly, it is difficult to talk to people in charge there. A few years ago, they were interested in MPTCP, but it was not available in the official Linux kernel. Now it is, and easily accessible (especially for small actors)... but Google has enough resources to find and use alternatives they fully control.
But the best is to let the app (and their users) controlling that, with a nice option. With Chrome/Firefox/..., we could enable MPTCP per domain for example.
Of course you can do that. There are different timeouts (MPTCP level, TCP (and SSH) keep alive, etc.) to prevent having dangling connections for a while, but they can be changed if needed.
Note: if you don't see the two large graphs at the top, disable ad-blockers and/or try with another browser.