I just looked at goals per game data in the NHL. There was a long rise from around 1962 to a peak about 1980-1986, a small decline after that with a big decline around 1990 and a small rise around 2002.
So it seems the goalie/crease changes in 1991 may have had a big effect, but scores were declining anyway at that point.
I think, in addition to rule/equipment changes, and far better standard goalie play, also the game has overall advanced (with international players and more advanced coaching at the NHL and lower levels), so defenders miss fewer assignments, forwards defend better, etc. In addition to just better defensive play overall, a more advanced game means there’s less of a gap between the best and worst defenders (and best and worst goalkeepers) so forwards have fewer overall weak spots to pick on. And there’s less of a gap between the worst and best forwards, so there will be fewer record-setting goal scorers (even if overall goals scored remained constant).
Plus, another cultural/rules change is that there are now fewer third/fourth line enforcers who weren’t expected to score or defend well, so there are fewer weak defenders to take advantage of and more strong forwards to spread scoring among.