I went to a fly in yesterday in my Piper cub with another friend in his cub. No electrical systems, only handheld radios. I radioed we were a flight of two cubs, 5 miles west. Another pilot kept saying I don’t see you on my ADS-B. Well of course not, we don’t have electrical systems.
There are a lot of pilots that think everyone shows up on their TCAS, panel mount avionics, or iPad, but especially around busy GA airports that is often not the case. And remember, if the aircraft was originally certified without an electrical system you can operate in the mode C veil of class B airspace without a transponder or ADS-B. There is a group of us that does this regularly around the west side of the Washington DC Class B.