MQTT Transmission supports custom tag properties to enable certain features. Creating custom properties on MQTT tags is a simple process and is no different than custom properties on any other Ignition tag (as documented here).
Custom Tag Properties
Action: Controls whether or not a tag is included in Transmission Sparkplug message payloads.
This custom property will not work with User Defined Types (UDTs).
Action: Controls whether or not a tag is stored in a configured HistoryStore when Transmission is offline.
This property works together with the Enable History Storage by Default property set in MQTT Transmission under the Settings > Transmitters > History Settings tab. If added to a tag and set to the reverse of the global parameter, it will override the global setting for that specific tag.