https://www.theverge.com/2018/5/30/17410802/google-calendar-add-note-changed-events

A new feature has been added to Google Calendar that allows you to leave a note when changing an event, as reported by TechCrunch.

Starting today, if you change or delete an event, a dialog box appears where you can leave a note to say why the change occurred. If you choose to write something and send it to participants, the message will then appear not just on the event page within Google Calendar, but also in the email sent out alerting everyone to the updated event information.