I would rephrase it to "Note that neither very old devices (introduced up to around ~2011) - nor newer devices (introduced ~late 2015 or newer) can show unicode maps. Newer devices can however mostly be made compatible by using grey market firmware. (actually the firmwares that are designed to circumvent new garmin devices to show "pirated" copyrighted garmin maps will also mean that those devices will be able to show mkgmap unicode maps. In my opinion in Europe it would be legal to use such firmware if you simply want to show OSM based mkgmap created "non pirated" maps - while illegal if used to show "pirated" garmin maps.
The only new devices for which this firmware is not available/ or better the firmware patcher for this does not work are some of the garmin smart devices like the Fenix watches and I'm not sure if it works on all automotive devices like Zumo or Nuvi.
So maybe in this firmware patcher some clues could be found (or better if the author would release the relevant clues/sourcecode on request)?
(just google garmin firmware patcher to find it)