Thanks @Dariusz_Duma, @Morgan_Hardwood and @gaaned92 for clearing up the current state of affairs.
I had originally started using rawtherapee-unstable based on comments in this thread:
Back when that thread was posted (May 2016), I think the advice to use rawtherapee-unstable probably caused a bunch of people to become de facto testers. Just add the PPA, install the package, and let the updates install whenever the updater runs. I’m pretty certain more people were running the code that way than would have been if they all had to manually download and build from source.
The present situation (rawtherapee-unstable being older than rawtherapee) means rawtherapee-unstable is no longer fulfilling that role.
Personally, I see some benefit in there being an -unstable build just to get more (unwitting?) testers aboard. 