Understood, thanks for the clarification! I haven’t been an opt-in developer for a few years now, so the situation is a bit foreign to me!As a user, it's optional. As a developer offering FB/Twitter/Google, it's mandatory.
From Apple's own App Store requirements
So if a developer had these sign in services, they needed to add SIWA, which has different mechanisms that devs need to support.
Cheers!