I notice in the menus of the Canon 430EX III-RT the firmware version is shown (mine shows "Ver 1.0.1"). Is this just for diagnostic purposes? Or, is it possible to update the firmware? I can't see any obvious way for a user to update the firmware as there's no USB port. So, if it can be done, I'm assuming Canon would need to do it. Just curious.
Generally, when there is an issue between a Canon EOS camera and a Canon flash, if possible Canon will issue an update for the camera to work around the issue. I'm not sure I've ever seen a user accessible firmware update for a Canon flash.
There have been recalls/updates that require sending a flash in to Canon to fix a bug.
This advisory regarding the firmware for certain serial number ranges of the 600EX-RT and ST-E3-RT requires shipping the unit(s) to a Canon Service Center for inspection and service. When that advisory was issued over three years ago, Brian Worley questioned why Canon does not make flashes updatable by the end user.