We need the ability for the community; either counsel, or forum administration to set a project as permanently abandoned.
What if the original developer unexpectedly returns after that and starts updating the project?
The only foolproof way to tell whether or not a project is fully abandoned is to either ask the developer(s), or to have proof the developer(s) will never be able to work on it again[e.g. due to death or an untreatable condition interfering].
Due to human error any other way will eventually falsely flag a project as abandoned, unless I overlooked a non-arbitrary way to tell, in which case please point it out.