This is sometimes problematic. You can try this:
-on a server smart group that lists all servers right click and choose ‘set property’
-choose the value and click on the blue circle arrow (default)
-try to delete the property.
-if it still wants to deprecate,
-run database cleanup
-try to delete again
If that doesn’t work, the only option is to deprecate.
It is not only a matter of if it is currently in use on servers. It could also be referenced in job runs, etc. All you can do is deprecate, in this case.