Did you have to do anything to make universal control work? I guess I just assumed that since it was a new feature all of my old Apple products wouldn't work (like my 2012 Mac mini that runs a family dashboard doesn't work with UC).
Looks like you need a Mac from 2015 or newer and an iPad from the same year. Depends on the model though. Similar to Sidecar, I think it depends on video encoding/decoding features introduced in Skylake chipsets, along with some newer version of Bluetooth.
There might be some hacky stuff you can do to get it to run, but I wouldn’t expect good performance or reliability.
You could always run the iPad with calendar/reminders in split screen with no Universal Control - if you have a gorilla arm.