Read Only Attributes with Legacy User Profile Provider

I’m using Keycloak 19.0.1 and --spi-user-profile-legacy-user-profile-read-only-attributes doesn’t seem to have any effect.
In “Realm Info” I see “DECLARATIVE_USER_PROFILE” under “Disabled Features”, in “Provider Info” I see “userProfile declarative-user-profile”. I assume that means that the old “Legacy User Profile Provider” is not active anymore.

Assuming that I want to have the old behavior for the time being:

How can I reactive the “Legacy User Profile Provider”?

I tried:
--spi-user-profile-provider=UserProfileProvider
--spi-user-profile-provider=LegacyUserProfileProvider
Also in lowercase and with dashes.
In combination with
--spi-user-profile-legacy-user-profile-enabled=true
and
-spi-user-profile-legacy-user-profile-read-only-attributes=...

Whenever the provider name is wrong I get HTTP 500 when I access the user profile.
--spi-user-profile-provider=declarative-user-provider works, but is not what I want, for now.