While you can find a plenty examples before outside of WWE such as Horseman, Flair etc, I would say Survivor Series 96 Sid entrance was the first time or at least first time on a grand stage, you had a straight up non-cool Heel cheered, mostly based on a backlash or hate for the babyface being shoved down their throat.
I would also say the WM20 match with Brock and Goldberg was probably the beginning of crowds taking over a show and doing chants and making it about themselves. I'm not saying this was the first match with chants but I remember the crowd being super pissed outside of a kayfabe level and being mad about both of them leaving after the match and completely sh*tting on it.
interestingly enough, both happened at MSG