I think that @theomegabit@infosec.exchange is asking for Proton to become an OAuth/OIDC provider. This would allow you to sign into any service, app, platform, etc. that supports it using your Proton account. Some common providers that are widely supported are Google, Apple, Github, Facebook, and Microsoft.

It is generally considered more secure than using "regular credentials" like username/email and password when using several services. There are a few downsides to this though. One of those downsides is that your OAuth/OIDC provider will have record of all your accounts used through OAuth/OIDC. For example, @theomegabit@infosec.exchange would like to avoid Google knowing about the various services used.