I see that

AuthenticationContext.principal

is now deprecated. What if I don't know the provider of my principals and I just want to get a list of them? That doesn't seem possible anymore?

You can omit a provider’s name and just specify the type of a principal.

Sorry, what I meant was, I don't know the type. It could be one of several types. My workaround right now is an extension function that goes through the possible types and attempts to return them e.g.:

fun AuthenticationContext.somePrincipal(): Principal? =
  principal<JWTPrincipal>()
    ?: principal<MyCustomPrincipal>()
    ?: principal<AnonymousPrincipal>()

but that seems to be more work than is necessary when before I could just do:

when(val principal = authentication.principal) {
   ...
}