I'd be more worried about why they are dropping their chain than whether or not they have a chain catcher. Though if this is a Saturday ride from the bike shop where many noobs will be likely to be attending. Then is it mainly the noobs that are dropping chains? Or people with a new to them electronic shifted bike?

Noobs to derailleur equipped bikes will drop a chain. And at stops I'd suspect they were downshifting just prior to the stop and didn't complete the shift before fully stopping. Similarly, even I had issues with my Di2 at first as I had to learn not to touch the shift levers when I picked up the front of the bike to reposition my wheel and other such silly things that with cable pulled shifting wasn't a issue as there wasn't enough force or movement to cause a shift. However for electronic shifting, it only needs a bump of the button. And that also can put the chain in a position to drop when the crank is turned backwards at a stop. As many of us do when we are winding up to go!

If you are going to mandate a chain catcher. Then there are all sorts of other things we probably need to mandate too! Where will it all end? <grin>