Scala 2 has something like this: there are the nightlies, and then there is the version given at https://github.com/scala/community-build/blob/2.13.x/nightly.properties . This isn’t just any nightly, but a nightly that is known to have passed the community build. It usually only changes every 2 to 3 weeks. (Sometimes more often in the runup to a proper release.)
We have never really publicized this much, and there isn’t really any tooling around it, so people don’t know about it. Regardless, it’s out there, fwiw.
(GitHub - dwijnand/scala-runners: Scala Runners: a Coursier-based alternative implementation supported it — it offered both 2.nightly and 2.next — but it got obsoleted by scala-cli.)
If Scala 3 were to develop a notion of special “testing” nightlies, or whatever terminology we want to use, and if tools such as scala-cli supported it, that same tooling support could also take advantage of this existing Scala 2 thing.
While I’m on the subject, I’ll note that a difference between the 2 and 3 ecosystems is that compiler plugins are popular and common in the 2 ecosystem, but aren’t (yet?) in the 3 ecosystem. Scala 2 nightlies are often difficult to use in real projects, since compiler plugins aren’t available for them. It usually works to use the plugins that were published for the last minor Scala version, but not invariably, and there isn’t any tooling support for asking for “previous minor Scala version” for the fully-versioned dependencies in your build.