Why are scheduled events not appearing on calendar?

The help page has been moved here.
This page is not updated, so please refer to the above for the latest information.
Following a specification change in Google Calendar in February 2023, there's a chance that confirmed events won't automatically appear on your Google Calendar. If you get an email similar to the one shown below, simply click the 'I know the sender' button to ensure it's added to your Google Calendar.
To always add to Google Calendar, in your Google Calendar event settings, set the "Add invitations to my calendar" option to "From everyone" for proper display.
If you choose "When I respond to the invitation in email" option, it won't add until you respond to the invitation email regarding your attendance.