A streamer made a comment about this that made sense right 6-8 years ago about why LFG/LFR is considered anti-social.
Before them, you had natural segregation of players, and its the things you go through that turn you toxic, apart from some weirdos that are toxic by default cause some another mental illness is happening there, but i doubt those were a lot.
Before the dungeon finders, someone that played the game more, was better or whatever you wanna call it, had no reason to play or ever meet someone less skilled in PvE content.
In Vanilla, you knew the raiding guilds, the most progressed players, you filtered and segregated based on that, in TBC and Wrath the same, it doesnt mean those players were good, but you had a pool of players to choose from.
But as example, someone more dedicated to the game at the time, would never meet someone that would take 8 months to reach max level, or someone that was raiding Black Temple, would never meet someone that was running Karazhan 1 year after and so on.
The fuck up, and why the system (RDF) is problematic is that Blizzard added rewards behind those systems which forced those players to meet.
Eventually you will have toxic situations because its completely different worlds and skills of gaming, that they SHOULD NEVER MEET.
I have gotten kicked in Wrath dungeon finder "for going too fast" on Utgarde Keep and "ruining the experience" still baffled about that 13 years after , the rare occasions i am not a tank or a healer to level fast on alts, i saw 3 times of people complaining about "ruining their experience" because of geared mains boosting their friends on leveling dungeons, both in BFA and SL and how WoW is ruined and it shouldnt be allowed.
I mean, i have seen some of the calmest people i know that never chat, actually go haywire in chat and Alt+F4 by situations in the last 13 years in RDF cause they simply cant be bothered.