Yet, any form of Enhanced Mode S is explicitly banned by EASA, on aircraft on which it is not mandatory
No, it is not banned. However the technical and therefore financial barrier is high enough to make it impracticable. However, Enhanced Surveillance delivers more information than needed for simple ADS-B out (except the position of course). ADS-B out can be achieved pretty cheap currently. As can be a simple ADS-B in traffic display. For example powerflam is available for less than 1k€ and displays flarm data as well which is used in over 18.000 glider and GA airplanes worldwide, most of them in europe.
Isn't the Mode S extended squitter (1090MHz) required above FL240 in the USA for ADS-B? Which really does not matter for most GA airplanes of course.