I am 40 yrs old and English is my second language. I cant give you some specific examples but when I watch old hollywood movies (50-60s), I mostly understand dialogues. Dialogues from modern movies are a bit harder to understand and I cant watch any movie without subtitle.

I think apart from increased desire to be perceived as realistic today, in old times there was a stronger need to articulate speech clearly, because of constraints in recording and playing audio.

Then conversely, today it seems that filmmmakers often assume their series or movies are consumed on headphones or loud TVs

Maybe sound compression ? I need to bump audio to 80 % to underestand anything on Netflix.

A film not a show, but Tenet is notorious for this.