I don’t think they are illegal. Just think that clubs want everyone on digital or whatever the new stuff is to keep channel spacing and prevent interference. AM/FM would only allow a certain number of fliers at once and anyone could transmit and crash you.
The new radios are spread spectrum and have PCM. Seems like I went to a fun fly somewhere once and all old radios were banned.
Literally the same comparison as using a ham radio for personal communication instead of a cell phone.