I received an email saying that some of my cron monitors were muted or disabled, and I’m not sure why I got this message or whether it’s related to billing, usage limits, or monitor alerts.

The “X of your Cron Monitors has been muted” email is a cron-specific notification sent to all members of the monitor’s team when alerts are silenced. These emails can be configured here, under the category "Broken Cron Monitors".