I finally managed to disable the "Recents" folder/search. I'm running High Sierra 10.13.6 Beta, but it probably is the same on all versions that have the Recents feature.
I haven't seen any side effects, but, of course, you're playing with the system, so the usual caveats apply - make a backup, no guarantee it won't break something or that it'll work for you, make sure you're comfortable at the system level, etc., etc.
1. Disable SIP (Google will turn up lots of sites that tell you how to do it), and reboot.
2. In Finder, "Go/Go to Folder" /System/Library/CoreServices/Finder.app/Contents/Applications/ and delete Recents.app.
3. Go to /System/Library/CoreServices/Finder.app/Contents/Resources/MyLibraries/myDocume nts.cannedSearch and delete the copy of Recents.app that's there.
4. Re-enable SIP.
Recents will still show up under the Finder's Go menu, but it'll be greyed out.