Spam reporting is pretty standardized? If your email client doesn't support it that's not Google's fault.

edit: I might be incorrect on this and was thinking about how unsubscribing is standardized instead.

Standardized how?

Basically, there is no standard beyond the ages-old requirement to have abuse@ and postmaster@ email addresses that react to such reports. Which Google doesn't follow at all, you just get redirected to some useless web form which requires a Google account and the sacrifice of a goat.

It is entirely Google's fault, and they should be shunned for it and their emails dropped. But unfortunately, they are too big for that by far...

Marking a mail as spam locally is different from spam reporting