Nudge is a piece of software which encourages the user to please update their outdated macOS. I suggest you read (lots) more about it on the Nudge wiki and elsewhere on the interwebs if you aren’t as familiar with it as you would like. Suffice to say, Nudge should be configured to suit your organisation and part of this configuration changes with each released version of macOS. As goes with any Admin tool, Nudge should be configured as kindly and thoughtfully as possible.