The Google calendar CalDAV server interface no longer supports username&password authentication, but only Oauth2. (https://developers.google.com/calendar/caldav/v2/guide)
Could this be implemented in the plugin, and what would be the best way to do that, or help with it?
Thanks for the report. I do not use Google calendar, so I never had the necessity to implement it (I only sync against Nextcloud).
Of course, this could be added, but requires quite some modifications of the webdavclient PlugIn. Specifically, all handling of how the HTTP client is set up needs to be reworked to support OAuth.
Currently, I do not have the necessary resources to work on this, sorry. If you would like to try it yourself, I suggest having a look at Google's API client: https://developers.google.com/api-client-library/php/auth/web-app. If I were to integrate that, I would probably add a Dropdown to the Admin interface to select "OAuth" or "Username/Password" when adding a new connection.