So this is far from scientific, but I was at a shopping center I used to go to a lot as a kid recently and I noticed that the sidewalks were free of gum. As a kid I remember it always being covered in discharged gum, i used to talk about it with my mom, the sidewalk was practically polka-dotted back then.

Of course there's a lot of possible explanations besides people just not chewing gum anymore. Could be that people have gotten better about disposing of gum properly, newer gum formulations could be easier to clean up, or we've gotten better at it how we clean it, or there's the fact that teenagers used to just kind of hang around outside of stores and don't/can't really do that so much anymore so there's less people loitering around and spitting their gum out.