To be clear a FeatureRequest not a Bug. So I think the age is understandable.
PRs:
Most recent: https://github.com/mobile-shell/mosh/pull/696
This is hard for open source projects in general. What it needs is not just a happy user base, but also developers and a strong leader wanting to move this into the future. I think specially the latter is missing. I think Mosh was a great thesis project, but stalled after that.
It has so much potential. I wish they would push this to the next level and do proper protocol documentation and an IETF proposal like an RFC.