I’m in the same boat! It’s not clear. I found this on another site talking about iOS14 Beta… But no clue yet about whether they’ve pulled an Aperture/iPhotos move on us folks with Reminders…
I am on iOS 14 and still on Mojave. Confirmed: you can still retain the legacy reminders format - it is not a forced upgrade. Good thing, as I’m almost certainly skipping Catalina, and am holding off on Big Sur for several months - likely will eventually put together a Mojave VM to run some older 32-bit software, but I’m in no rush.