My guess is that modern chess players take good care of their health and don't smoke or do things that could jeopardize it. Back then they didn't know or understand how such things could affect health. That old reputation of them having bad habits and not taking care of their health probably had more to do with the bad ones sticking out like a sore thumb among otherwise fastidious gentlemen. It also wasn't helped by the fact that most were soviet-based, which used to have a terrible culture of drink and damaging behaviour (a relatively low life expectancy of just 64 for Russian males persists to this day).
Smoking chess players

I smoke like a chimney and love it. I doubt I could sit through an OTB without having to go outside.
But on the whole people are definatly smarter about their health and body. Mentally and physically. Which is a good thing.

I play strictly online (either 5 or 10 minute matches), on my laptop on my back porch- I play online only because I have just started playing about a month ago and am hardly worthy of the 400 level I currently hold. So yes, I smoke during almost every match I play. However, I seriously doubt the prevalency of smoking is anywhere near what it used to be- due to increased awareness of the health implications. I have lived a life full of vices, and smoking is relatively low on my list of things that can have a negative impact on my health, especially when considering how immediate the impact might be. Although, on a darker note, I consider smoking to be a sort of "insurance plan" to avoid the plagues of old age- but that is more of an issue with my depressed outlook on life than anything else.
Do any chess players still smoke nowadays ? 20 years ago, the chess clubs were like smoking rooms. Tal used to smoke, but then he died at the relatively young age of 55.