OpenSSH 9.5 released

A longer comparison: mosh uses 0-1 connections, mptcp 1-∞. Mosh can survive a total disconnection, although that protects you only from losing the session's contents, you still sit there doing nothing until you manually deconfigure your landline. MPTCP on the other hand requires at least one flow to work all the time, but hops between them with very low latency. All you notice is a small lag spike.

Mosh is also limited to interactive sessions only. It can't scp, can't tunnel, can't jump host, can't X forward, etc.

It's also a separate program that uses ssh only for initial setup (authentication, setting up the two UDP endpoints) -- after that, mosh doesn't interact with ssh at all. MPTCP on the other hand is a generic TCP extension that's usable with all programs.

They have distinct use cases, just with an overlap for roaming.

and unlike mosh, you can also have functional scrollback, which is kinda useful