Another Part 15 option that has existed, at least since the early 1970's in the USA, is Cable FM Radio.
https://en.wikipedia.org/wiki/Cable_radio (I'm sure there are other explanations out there, but that was from a very quick search.) Anyway, in the 70's, 80's, and the 90's, college campuses and small towns, with CATV companies, could allow a station to exist via only being available on their cabled system. Where I grew up, we had Cable FM, but not anything local access like, just re-FREQ-assigned of rebroadcasted stations from Buffalo, Rochester, Hornell, Olean, Niagara Falls, in WNY, and even Toronto, ON, Canada. Local FM reception was dismal, at best, in that area, so FM was nice for the cable company to offer at the time. The only thing that was Cable-Only was the leaky-cable coo-coo at the very start of the FM dial. That, was the tool where cable company employees could drive around, using just a car stereo's FM RX, to find out where the RF leaky cable faults were, and, or, see whom was stealing CATV illegally. Bottom-line, does anyone know of any Cable FM only stations that exist only on the local CATV system, that they are carried on? Or, do you have stories of such station in the past?