I remember that in 1970 Congress banned advertising cigarettes on TV - but characters on TV shows still continued to smoke. I don't remember when seeing TV characters smoke pretty much came to a stop.
The funny thing is, I don't remember Stokes ever smoking on the daytime show. Did he? But so far as this film goes, maybe in real life Thayer was an activist for smokers' rights and he was damn well going to smoke on the movie screen. Or maybe he wanted to introduce or play up one of Stokes' idiosyncrasies. Or perhaps it was as simple as smoking was a way for Thayer himself to calm the tension of making the film...
I seem to remember Timothy Stokes smoking in parallel time and I think that Eliot did in the haunting of Collinwood, where Quentin comes close to killing him. Julia, of course, was a chimney at times. And it was kind of a plot point with Cassandra. Don Briscoe said in an interview that he didn't want to smoke on camera but a certain someone insisted that he did.
The only character where it seemed to make perfect sense to be a smoker was 1967 Laura.