major distros are maintained, and they wouldn't be shipping it if it had bugs and/or was being used as an exploit
i.e. this package being somehow abandoned and therefore should not be trusted is likely to be false
> i would imagine that distros are not keeping important patches like security to themselves.
I'm not 100% sure what "keeping to themselves" means in context of GPL 3 code, but one can verify with the mosh GitHub link to see the upstream project has not had a single commit on any branch for the last 2.5 years.
The project is dead, it's up to your trust+verification of any specific downstream packaging as to how much of a problem that is for the binary you may be using. Some maintainers may not have noticed/cared enough yet, some maintainers may only carry security fixes of known CVEs, some maintainers may be managing a full fork. The average reader probably wants to note that for their specific binary rather than note Fedora still packages a downstream version (which may be completely different).