It is possible for users to add you to a Watson Studio project, however, after a while you can end up belonging to a lot of other projects.
How can you remove yourself from another project? Is it possible to do this, or do you need to contact the project owner and ask them to remove you?
It is not possible at the moment. You already did the right thing by raising an uservoice suggestion. I am going to upvote :)
Update: This is possible since some time now. Go to the list of projects and click on the 3 dot menu --> Leave Project