See https://github.com/google/google-api-javascript-client/issue...
Chromium bug marked WONTFIX: https://bugs.chromium.org/p/chromium/issues/detail?id=116445...
Updated readme: https://github.com/google/google-api-javascript-client/blob/...
So effectively this means extensions on MV3 can't easily access Google apis, which is quite unfortunate since Chrome extensions in particular made Google authentication super straightforward (piggybacking off of chrome's built-in google authentication). If someone knows a better way I'd love to hear it.
I believe the reason that the current incarnation of the javascript library won't work is because it modifies the dom to add script tags to fetch and run the api library (or components of it), which is specifically what MV3 will disallow AFAIK.