Nevermind. Did some digging and the information is no longer stored in a .plist but rather a .json located at ~/Library/Application Support/SketchUp 2018/SketchUp/SharedPrefereneces.json
Of note, in that file you can define “CheckForUpdates”: false, and “RubyManager_DisablePlugins”: true which is exactly what I was looking for.
Since our Macs are bound, I needed this file manipulated per user, so I created a LaunchAgent to run a script which basically creates the customised SharedPreferences.json file while the user logs in.
Not as pretty as manipulating these settings silently with profiles as in the past, but it gets the job done - and silently, at that.
Just thought I would share this if anyone else happens to come across this obstacle.