https://techcrunch.com/2018/05/30/google-calendar-now-lets-you-add-a-message-when-you-change-an-event/

Google is adding a small but useful feature to Google Calendar. Starting today, when you change or delete an event, a dialog box now pops up that allows you to attach a short message to the event to explain why you are making the change and what’s changing.

Here is how this new feature will work: When you make a change, a dialog box will pop up and allow you to enter a message for your guests. On the event page in Google Calendar itself — and in the email that alerts your guests of the change — that message will then appear at the top of the event details section.