How does the alerts feature "Send a notification if data is missing for X minutes" work combined with grouping? For example, I'd like to monitor 50+ different client websites and want to be alerted if one of them stops sending data, but I'd rather not set up to 50+ separate alerts for this. That said, I'd also like to officially acknowledge missing data and sunset a specific website

It doesn't work for grouping. You can use promql for this.