Need to change recommendation.
See:
https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901246

4.7.2 Topics beginning with $

The Server MUST NOT match Topic Filters starting with a wildcard character (# or +) with Topic Names beginning with a $ character [MQTT-4.7.2-1]. The Server SHOULD prevent Clients from using such Topic Names to exchange messages with other Clients. Server implementations MAY use Topic Names that start with a leading $ character for other purposes.

Non-normative comment

· $SYS/ has been widely adopted as a prefix to topics that contain Server-specific information or control APIs
· Applications cannot use a topic with a leading $ character for their own purposes