I suspect something like 'last login date by user' or 'last edit date by user', just need something to show people are actually using the software. Nothing overly complex needed.

ideally this is something we would be looking for periodically, I imagine annually at least. I'm surprised this isn't something you get a request for more often, from enterprise scale customers. I suspect it is something that would be of interest to them for use by their software asset management teams for ongoing license recovery activity.