No the 19 seat commuter category came from the other side of the Atlantic long before EASA had even been envisaged!
I was rather thinking of the reason for EASA using "19" in its definition of "complex".
It doesn't catch much in Europe, but protects Pilatus while enabling Beech to be screwed.
The "complex" definition crucially also appears in the requirement for
EASA maintenance, for foreign-reg planes. So, an N-reg TBM or PC12 based in Europe can be maintained under FAA Part 91, whereas a King Air 90 will need both Part 91 (to comply with the State of Registry requirements) and an EASA Part M signoff.
How the EASA signoff is supposed to work I have no idea because an EASA 145 company has no legal competence on non EASA reg planes, and what happens if the aircraft has any FAA STC mods which are not EASA STC'd? It's completely meaningless.