Why another Google synchronization add on - zanonmark/Google-4-TbSync GitHub Wiki

Google currently offers 3 2 ways to perform synchronization of contacts:

1. Through the CardDAV protocol. This should be the preferred choice, because it's a standard protocol and there are already many interfaces for it. But the support by Google is partially broken, as it doesn't support synchronizing contact groups (aka labels). So, this is a no-go unless Google fixes it (which seems to be: never, as the ticket has been open for years).
2. Through the Google Contacts API. This is what the gContactSync add-on was doing. But: a) the user interface of gContactSync is currently broken, and will probably not be fixed; b) even if it was, the Google Contacts API itself is deprecated and will stop working on June 15th 2021.
3. Through the Google's People API. This is exactly what the Google-4-TbSync add-on was developed for.

To synchronize calendars, any CalDAV-compatible software can be used instead, like Thunderbird itself or one of its add-ons. So Google-4-TbSync is not planned to offer such a support.