This process should include end-of-life requirements: If you stop maintaining an extension, you should post a plan for a graceful removal/information about when support stopped (e.g., "last tested on this version of MediaWiki").
Topic on Talk:WMF product development process
End of life
Good point. I guess this would belong to the Maintain stage.
It should probably be its own stage. Everything, even something as durable as Microsoft Windows, is likely to have an End of Life stage some day.
It is a good point. A Retire stage could be a nice addition. What types of tasks and stakeholders would be involved?
For tasks, I suppose you'd need to figure out any dependencies (both directions: Did you add something else to support this product, that could be removed with it? Is anything else relying upon your code?) and make suitable announcements and updates to documentation.
For stakeholders, I would guess this short list:
Product Manager – to make the decision
Community Liaisons – to find out whether anyone at the WMF wikis is using it
Operations – to deal with practical stuff about removal
Just FYI, I have added this discussion as blocker of Phab:T125806 (Consolidate the WMF product development process overview).