You are correct that this was not previously restricted. This was put in place to conform to the OIDC specification. Note that if you are not using OIDC/OAuth this won’t restrict your rules, its only necessary to namespace in OIDC.
If you are, both authentication pipelines (OIDC-conformant and legacy) will be usable until further notice. Currently, there is not a date in place when this will absolutely need to be changed for older tenants. However, we strongly recommend that you update the namespace identifiers to be OIDC-conformant in the near future.
Please let me know if that answers your question or if you have any further questions.