[lategloriousgnu]

Ah, on further reading it looks like there are a couple parts to it.

I have only used the "Sign in with Apple" feature directly in apps, which only ever lets you create one for that app.

However, apparently with an iCloud+ subscription, you can generate arbitrary email addresses from within iCloud itself, and then use those wherever you like.

https://support.apple.com/en-au/HT210425