My understanding is that the upgrade to Catalina gives you access to your reminders if you have upgraded your reminders. I haven’t done that.
I confirmed with Apple support as recently as yesterday that Apple is aware of major problems with Reminders and that fixing those reminders is a top priority. I agree, though, this is a major screw up on Apple’s part. On iCloud.com, all apps work except reminders. Clicking on reminders on iCloud.com crashes.
I’ve started deleting some of my 9000+ completed repeating reminders from one of my Macs to see if that will have any effect. Reminders seem slow on my Mac but otherwise work.
They are a mess on my phone. They take forever to sync and the ‘mark completed,’ ‘ignore’ etc options just don’t work. Im missing important things like medication reminders so I’m pretty unhappy.