Is Notes sync broken on iOS 14 and macOS Mojave 10.14?
I'm using iOS 14.4 and macOS 10.14.6 and notes actually sync better as compared to iOS 12.
Keep an eye on apple forums, release notes etc in case other iOS updates break sync.

https://developer.apple.com/forums/
https://developer.apple.com/documentation/ios-ipados-release-notes

Downgrading iOS version from backup seems not impossible as per Can I install any iOS version I wish by picking and choosing which firmware to restore?
So wait for a few weeks before upgrading iOS and see if people are complaining about sync problems.