Full time touring IEM mixer and kinda retired audience taper here.
Not sure how it used to work in the past, but I would say it is harder to do nowadays, especially stealthily
Most of the bands that are using IEM's are on Shure PSM1000's, which have encrypted transmission and receiving codecs.
Since IEM technology has moved past the exploratory phase and into mission critical broadcast, security, and other fields... it would be very difficult.
An encrypted transmission will show up on a RF scanner but unless your receiver has been "synced" to the accompanying transmitter, you'll hear no audio on the other end.
Furthermore, I am all about taping shows, even stealthily. There are bands that are anti-taping that are delighted to receive an audience copy of a favorite performance after many years, even if the taper had to sneak gear into the venue. However I would be pretty annoyed learning that someone was making recordings of my IEM mixes.. alot of these mixes have talkback mics and click tracks and other things that the artist is not intending for the audience to hear. Personal requests and chatter with the techs is also something that wasn't meant to be recorded. Similar to recording a telephone conversation that was thought to be in confidence.
Also the physical act of getting IEM gear including scanners, etc into a venue in 2020 is way more difficult than it was in 2004.
Any other questions, ptmoore37@gmail.com. I have experience on both sides of this conversation!