Slack does, or did, do this. I believe Trello, too.
I found out about this because I noticed our Slack bill was quite a lot lower over some Christmas/January period. It was because so many folks were away, and so they didn't charge us for seats that were inactive for > 30 days.
Yes, lots of businesses charge based on MAU. You can pre-pay for a certain MAU, which will get you a lower price per user, but at the expense of paying even if they aren't used. Which is fair enough.
I found out about this because I noticed our Slack bill was quite a lot lower over some Christmas/January period. It was because so many folks were away, and so they didn't charge us for seats that were inactive for > 30 days.