Because of an API change the birthdays are not provided anymore as a separate calendar, but these events are added to the regular calendar.
However they can be requested on basis of the eventType.
Google Calendar API supports get, instances, and list methods for reading birthday events. eventTypes can be set to 'birthday' to list only birthday events. If no type is specified, birthdays will be listed alongside all other event types.
Request is therefore to implement the birthdays as a separate calendar in Home Assistant itself, to bring back the old behaviour.
I noticed this today as well. I was so used to seeing Birthdays as a different color on my calendar rather than the same color as my gmail. This would be a nice.
Exported all the calendars from Google Calendar (this will export the birthday calendar as a separate file).
Created a new “Birthdays” calendar and imported the previously exported file into it.
Disabled contact synchronization in the original birthday calendar.
This way, I no longer have birthdays in the default calendar, and all the birthdays are in the new one. The only downside is that if I add a new birthday to my Google Contacts, I need to manually add it to the new calendar.