Multipath TCP sadly seems to have died... I believe it was incompatible with internal load balancing systems at some big companies, so nobody deployed it.
I mean, I haven't deployed it on my own servers just because it still requires a kernel fork to enable on Linux. There was an effort to upstream it as of late 2019 though; not sure if that's stalled, but I can understand it taking a long time as I believe it's a pretty big change to the Linux networking stack.
Apple deployed it in iOS and macos. You can access it as a developer of either.
There is a load balancing issue, but I don't think it's any worse than http/3 which also allows for peers to change IP (http/3 has a much limited scope of changes).