How can I disable the "no backup in x days" from TimeMachine?
When you don't have the external hard drive hooked up, go System Preferences >> Time Machine, and switch it to OFF. You will still be able to perform backups, it just won't happen automatically.

In newer OS versions, you must uncheck the Back up automatically box and then right-click on the backup disk on the right side and select Stop using ‘Time Machine’ Backups.

This is the technique that I use, and I don't see notifications.