REMINDERS in Mojave and High Sierra once i upgraded iOS?

Unfortunately, the article is quite clear:

Upgraded reminders aren't compatible with earlier versions of iOS and macOS. If you upgrade your reminders on your iPhone with iOS 13 or later, your other devices — like your iPad and Mac — can’t access the reminders until you update your iPad to iPadOS or your Mac to macOS 10.15 Catalina.

The is no known way to revert the Reminder upgrade. If you can't upgrade your two Macs to macOS Catalina or later, you'll need to access your reminders through iCloud.com.

Yes, this is what I mean. The same article you mentioned states:

When you open the app, a Welcome to Reminders screen gives you the options to upgrade now or upgrade later. Reminders will also tell you if you have shared Reminders lists that will be affected, or other devices that also need to be upgraded.

Tap Upgrade Now to begin the process. If you tap Upgrade Later, the app's main screen appears and shows a blue Upgrade button above your lists. Tap it when you're ready to upgrade your reminders.

Yes, if your iPhone is set to update automatically and was updated to iOS 13 or later, other devices, iPhones using iOS 12 - or earlier - and Macs using macOS Mojave - or earlier - automatically loose the ability to use the new Reminders format.

Yes, the last one in mind was for the new Apple Home architecture (the Home app on iPhones and recents Macs). Once you committed to the new architecture, your other devices had to have a minimum OS requirements.

Just like the Reminders upgrade, you had to agree to the new Apple Home architecture. In the future, if prompted to agree to a new architecture/data format etc., wait a while and do some web searches to see if other users have issues with the new features.

Well, not exactly: after an iOS or macOS update, you'd be prompted to upgrade the architecture/data format when first using the app (Reminders or Home) after the OS upgrade. You can choose to decline until you are sure all your devices are perfectly compatible.

That being said, some users choose not to upgrade iOS or macOS automatically and wait to see if those OS's have any intrinsic issues (not related to new architecture/data format in apps).