OpenID Connect: Use username as subject without script mapper

By default, Keycloak provides the user id as sub(ject) in the OIDC ID token. But instead, I need the username as subject. I already tried the mentioned solutions for older, similar questions:

1. Using a script mapper (deprecated feature, not available in my environment / version 8.0.2 anymore)
2. Using a user attribute mapper (does not work)

Is there any other known solution?