I'm still on Mojave and it's okay honestly. It does everything I need my OS to do. I see no benefit in upgrading to Catalina, especially with it not supporting 32-bit apps and introducing new annoyances under the guise of "security", and Big Sur doesn't (officially) run on my computer at all.
Granular app permissions are a huge thing for me; I really like being able to keep certain apps from having e.g. full disk access or camera access. Plus sidecar, as sibling already mentioned.
Mojave has some of these. Except I don't like filesystem permissions, e.g. "access to photos" which is really "access to ~/Pictures". I'm fine with unix permissions on files and directories. I wish internet access was a permission instead.
As far as Sidecar goes, I don't own an iPad and don't plan to, so not relevant to me by any stretch of imagination.
