I finally decided I was done being sad looking at the name of my covid-casualty former band in my projects list. I went into the web app, into that project, batch-added every song to my new band project, and then went to Account > Projects to deactivate it.
I tried from the list/batch direction but for some reason it wouldn't show as deactivated. I drilled into the project detail and found the "Active" box checked and greyed out, so I physically couldn't uncheck it. None of my other projects were this way.
Then I realized I was IN that project. I changed to a different project and suddenly my old band's project WAS able to be deactivated. Makes sense--what would you do in the UI if you were suddenly in the context of a deactivated project? But I had a very confusing moment until I sorted that out.
Some sort of messaging to that effect would be easy and helpful, in this (admittedly, edge) case.