You can now use Sign In with Apple on Squarespace, the latest company to support Apple’s private sign in service. The Mac Observer keeps a list of other apps and services that use it.
Previously the Sign In with Apple was April 30 but in early March Apple extended that deadline to June 30, giving companies and developers more time to add it into their apps and services.
To find Sign In with Apple logins, go to Settings > Your Name > Password & Security. Then, tap Apps Using Your Apple ID. Tap on each individual app to see the details or tap the button to stop using Sign In with Apple with that app.