As a plugin developer myself, there are several reasons my I might want to orphan a plugin.

1) The functionality is now part of WordPress core.
2) I no longer have the time to dedicate to maintaining support for a plugin.
3) I lost interest in the plugin - it's hard to be motivated to develop a plugin you're no longer excited about :(

In the first instance, that's great, there's no reason to continue using the plugin. My approach to this would be to add a message to the top of the readme.txt file to say that it is no longer required since WordPress version X and update the plugin so it automatically notifies the user in the admin or deactivates itself if appropriate. See Compact Taxonomy Checkboxes for example - I have just updated with a more descriptive message :)

Having said this, there is no reason why someone may not want to take over the plugin and build on it further to enhance the WordPress core version of the functionality, so I think it's important to make it clear in the message that you are willing to transfer commit access.

In the second instance I would just add a message to the readme.txt as mentioned above so other developers are aware that I am willing to pass on commit access if someone else wants to continue development.

It would be nice if a plugin could automatically display such a message, maybe if you tagged it with "seeking-committer" or something?

That could then also be used to provide a list of plugins that are looking for people to contribute. That doesn't just have to be orphaned plugins either. I co-contribute on a few plugins after approaching the authors and would like to see more collaborating like this in the community. It may help to reduce the likelihood of plugins being neglected.

The main problem is those plugins where a developer seems to have just disappeared.

I have actually re-written 3 plugins which had not been updated in ages. For each I have made numerous attempts to contact the original authors through any contact channels I could find which included a lot of Googling and detective work, still without any response.

One of these I have since released as WP Mail From II, although it bugs me that I had to revert to doing this for such a simple plugin rather than the more practical solution of taking over the original plugin.

I have actually since been contacted by that author so I may now be able to get commit access, but it does remain an issue that people do just drop out of the community, sometimes to the extent that they are completely un-contactable.

I did contact WP.org to see if it was possible to take over the plugin and the advice was:

The best path right now is to fork the plugin. Submit the new plugin for hosting as 'wp mail from revisited' or something akin to that, and we'll review it and add it.

Sadly, unless the original author wants to hand it over, we won't do it for them.

I completely understand that. The principle of forking a plugin is fine but it doesn't help people who may be using that previous version of the plugin if that no longer appears to be maintained.

Maybe as well as showing the 2 years message, WP.org might consider pointing users to a fork of the plugin if one is created? (I'm thinking a proper fork of the old plugin, not just another plugin with similar functionality)...

...but then WP.org won't want to be seen to be 'recommending' certain plugins.

If it was more like GitHub, developers would be able to fork old plugins and old plugins would reference plugins that had been forked from them. Now there's an idea.....?