In versions of Keycloak using Wildfly (<17), this defaulted to /auth. If upgrading from the Wildfly version of Keycloak, i.e. a NixOS version before 22.05, you'll likely want to set this to /auth to keep compatibility with your clients.

See https://www.keycloak.org/migration/migrating-to-quarkus for more information on migrating from Wildfly to Quarkus.