On macOS Sequoia, the settings to enable FIFinderSync seem to have gone. I have already figured out that Extensions are no longer in the Privacy & Security section, but they are now at General › Login Items & Extensions. Here there is a Finder section, but that is just for the Finder-Extensions, not the Finder-Sync-Extensions. Those previously did not have their own section and were hidden away in the Added Extensions section that apparently no longer exists. I expect that it has been forgotten when migrating.
Where are the settings for this – have they been forgotten?