It is by design, I wish we knew this before we created islands for the dev team to be able to reproduce issues. I am afraid not every island can be repurposed, so for example if you start with RR template and decide to not proceed with it I do not think it is easy to convert the map to a normal uefn map. There is also this bug: I transferred a personal project to our team, but it instead created duplicate maps, team cannot play or publish
Also there are unrecoverable islands in disabled state which cannot be fixed even by administrator.
All these create clutter. Considering there are so many issues maybe it makes sense to allow creators delete islands after some time.