This becomes a more relevant problem with the new Siri watch face. I ended up deleting most birthdays in Contacts to stop birthdays from clustering my Apple Watch screen daily.
I tried the below but failed to achieve my goals:
- In the watch app on iPhone, go to calendar and mirror my iPhone's setting. I also make sure birthday is unchecked on the calendar settings of my iPhone. I confirmed this by going to my calendar on iphone to see no birthdays are showing.
- I then checked my watch, the birthday is still showing in calendar. I gave ample amount of time (15mins) for the Apple Watch to sync. Look again - the birthdays are still there.
- I then restarted my Watch - the birthdays are still there.
- I double checked my calendar app on iPhone, no birthdays are shown and the birthday calendar is still unchecked. The calendar setting in the watch app is still set at mirror iPhone.
Then I tried with using custom instead of mirror iPhone in the watch app->calendar:
Uncheck the Birthday calendar
The birthdays are still showing on watch
Wait 15mins - the birthdays are still here
So I ended up deleting most birthdays in Contacts and the birthdays stop showing on my watch instantly - except those birthdays I have kept.