Hello,
let me try to clear up the confusion about the Sculpt versioning.
Between any two official Sculpt versions, ABI breakage must be expected. For this reason, we increase the Sculpt version shortly after a Sculpt release so that a developer who publishes a package for testing purposes won't accidentally offer it for the latest official Sculpt release.
In contrast to Sculpt's official version 20.08, the unofficial version 20.09 does not refer to an actual version, but to an arbitrary in-between-two-versions state that is only meaningful for testing. Usually, when testing and running the staging version of Sculpt, one has to publish pkg/sculpt-installation packages under the developer's name to get a usable Sculpt system. In order to make the packages installable in Sculpt, one also needs to have a pubkey and download file located in <genode-dir>/depot/<user>/.
Unfortunately, the building and publishing of the pkg/sculpt-installation package is quite time consuming (e.g., building qt5, virtualbox, etc.). So it may be beneficial to temporarily remove these big packages from repos/gems/recipes/pkg/sculpt-installation/archives.
Fortunately, ABI breakage does not happen all the time. So Thomasz' suggestion to try the index of the other developers is certainly a good idea to try first.
Cheers Norman