The issue with advanced permissions is that there is no one-sized fits all option and it could become problematic in creating tons of permissions issues, an issue that doesn’t plague Airtable, unlike other software.
Variant/Portals are available from the Workspace Level, and you simply right click the base and “Create New Variant/Portal”.
i.e. a duplicate of the base that syncs… (to me, and I think others it would still be useful even if it was necessary to lock the base from having new fields if a Variant/Portal relating to the base existed.)
In my mind this is better than permissions as no user-level permissions need to be added to AT, just invite the correct users to the correct Variant/Portal relating to the correct base