Beta builds do have a finite lifespan. It is, as always, a mistake to use them for anything but experimental projects that may be 'broken' in various ways and have no lifespan past the edition that created them. (That is, a feature may not be released, or may be modified before release, leaving a project that uses an earlier iteration stranded.)
I think the only option is to wait for the beta features to be included in a release edition, or try an updated beta installation.
I'm getting the message as well. I had just tested 18.9 two or three days ago.
That usually means a new version is about to be released. These are TEST VERSIONS are not intended to be used in production.
I downloaded an InDesign cloud file that was saved with the (now expired) beta, directly from Creative Cloud and opened it succefully with the Pre-release version (it doesn't open with the regular version).
Definitely not a production flow but may be worth a try if you're really stuck.