Please take a look at this related issue in the web push libraries:
zaru/webpush#103
It doesn't make sense and it is against web standards to return a 406 for rate limiting. 406 has a very specific meaning.
This causes serious problems to the web push libraries, but it cannot be solved inside the libraries (because the libraries are correct and need to follow the standard).
Please consider fixing this wrong behavior and updating your documentation here:
https://learn.microsoft.com/en-us/windows/apps/design/shell/tiles-and-notifications/push-request-response-headers#response-codes
Please mention/tag the right team. Thanks