Weirdly enough I was able to work around this outage by using their new API endpoint URL api-m.paypal.com / api-m.sandbox.paypal.com instead of api.paypal.com / api.sandbox.paypal.com. No more 403s in the auth endpoint using the new URL that they use in their updated documentation: https://developer.paypal.com/api/rest/authentication
This URL change doesn't seem well communicated at all, but some people have gotten an email but there's no such details available officially anywhere publicly as far as I can tell.