csrutil disable
Restart computer
In the terminal, type
sudo open /Applications/TextEdit.app /System/Library/LaunchDaemons/com.apple.ManagedClient.enroll.plist
change
<key>com.apple.ManagedClient.enroll</key>
<true/>
to
<key>com.apple.ManagedClient.enroll</key>
<false/>
So that the changes take effect
The instructions for the main post are for Catalina and outdated. For updated current instructions for Big Sur and newer, please review the comments since Nov 24, 2020.
Summary: The easiest way to stop the alerts is to block network access to the Apple servers that report your Mac’s serial number is managed. Even after blocking the servers, some alerts may still appear for some time until specific commands or reboots are done. See the most recent posts for others going through the same issue.