Until they hit a hard bug or don’t support newer transport formats or scenarios. Also the community dries up eventually
That is why you test your backuo. It is unrealiatic, that in a stable software release there is suddenly, after you tested your backup a hard bug which prevents recovery.
Yes unmaintained software will not support new featueres.
I think you misunderstood me. You should not use unmaintained software as your backup tool, but IMO it is no problem when it suddenly goes unmaintained, your backup will most likely still work. Same with any other software, that goes unmaintained, look for an alternative.