At the very least Sign in with Apple needs to support a request for the user to enter the real email they want to use to be contacted by the app after-the-fact for cases where someone obfuscates their email but then wants access to functionality that explicitly requires their real email.
Having typed the foregoing, I realize now I'm basically just saying "don't use any third-party sign-in, including SIWA". Maybe it's fine that the ecosystem doesn't always cater to apps of questionable utility...
I assumed otherwise because they specifically wrote "sharing". Still, I'm not sure agree with:
> requiring them to provide a separate email from the one they registered with completely defeats the purpose of obfuscating the email in the first place
I can't use most apps without providing an email, whether they need it or not. That's a much worse situation than not being able to use some features without providing one (which still assumes me having no access to or knowledge of my own iCloud email).
But if you share a trip to a person who's on Tripit - they just get a notice and a record in their webapp.