As a plugin developer myself I am guilty of letting a few of my early lesser used plugins slip into abandonment and I've added a message to the readme files to make it clear the plugin is no longer being developed but would welcome anyone wishing to adopt it.

From the other side of the fence I have encountered quite a few plugins which would be of real value to me but which have not been updated in a long time or which have been abandon and are riddled with errors and don't work in recent versions of WordPress.

I have had moderate success in both adopting and contributing to old plugins by hunting down the author through various means starting in the forums, then moving to more detective work in Google and on Twitter to track down some recent contact details - a bit of determination and persistence can pay off.

I have adopted and been given joint commit access to 2 plugins in the repository and forked 1 for which I could not track down the owner.

I like the idea of an "adopt-me" tag but understand why this is not like to become official. No harm in using it anyway.