There are simple workarounds you can try to fix it, see for example https://github.com/linuxmint/cinnamon/issues/12994#issuecomment-3134995065

I don’t know exactly what part of a separate profile you are after, so this may not be a 100% substitute, but I found container tabs in Firefox to work quite well (with some extensions to improve UX). It’s still the same profile though, so passwords and history are shared.

If it is not indeed a new concept, it seems a great deal of people either didn’t know about it, or refused to care. Rather than be annoyed at the rediscovery, perhaps a better outlook would be to rejoice that these same ideas are reaching more people through the new words than it did with the old?