Since no one answered your questions....
Use this endpoint to download any final version of past major versions.
Where 234 = 2.3.4, etc... change to any version you want
/hub/advanced/downloadPlatform/234
Once downloaded it will be available in the Diagnostic tool under "Restore Prior Version..."
Was it referring to restoring a backup or restoring the platform? I suspect it may have been saying you cannot roll the platform back. The actual database backup can typically be restored.