How can I disable the Apple ID warning about not being signed into iCloud?
You could also issue this command in the Terminal:

defaults write com.apple.systempreferences AttentionPrefBundleIDs 0; killall Dock

This has worked for me so far (I'm on 10.15.2). Before that I created an alias for System Preferences, and put that in the dock instead. You get a small arrow at the bottom of the icon, but it's less eye-catching than the nasty red blob.