How can I downgrade Reminders as my Mac is not suitable for Catalina

Back in the day, I sat on so-called "Red Teams" where we were to question fellow developers about their code, the product intent, the user impact, the business impact, the support issue, and propose scenarios where the code would not do its job. I question whether Apple takes the time to do this, because if they had, who would have actually approved discontinuity between iOS/iPadOS, and macOS product lines?

Once you have upgraded the iOS/iPadOS 13 Reminders database, only Reminders on iCloud, or on Catalina can interoperate with the new format. You cannot downgrade Reminders.

I would agree.