Studio Pro 9.9.2 is available again on https://marketplace.mendix.com/link/studiopro/
Well it has happened with initial versions of 8 as well. Mendix usually takes them offline if they are unstable or have some serious issues. Best way would be to reach out to them to identify when a higher version will be made available.
But if the app is not in production then the support from Mendix could be minimal.
Best practice is to not use the latest version available that Mendix releases as its stability is not guaranteed in my opinion. Better to use LTS version which Mendix guarantees that they will support.