Why does Mail.app automatically add iCal events from invitations I receive?
There are web app preferences to manage this in newer macOS.
In the Mail.app go to Preferences->general. There is an option "Add invitations to ICal" which is Automatic or None. You need to choose None.
When you have a mail with date and time in Mail.App will highlight that and clicking on that will add an event to iCal.