Sort of.
I know a lot of people who quit, not directly because of it, but certainly indirectly because of it.
Mythic+, along with titanforging as it was in legion, caused a lot of players to burn out.
The idea of basicly infinately scaling difficulty and rewards, in a game where players strife for obtaining the best possible gear, ain't great.
But it's a sad trend that started way earlier, with the seperations of raids in wotlk.
You went from:
Classic: Quests -> Dungeons -> Raids
TBC: Quests -> Dungeons -> HC Dungeons -> Raids
WotLK: Quests -> Dungeons -> HC Dungeons -> 10man raids -> 10man HC/25man raids - > 25man HC raids
Cataclysm: Quests -> Dungeons -> HC Dungeons -> Raidfinder -> 10man raids -> 10man HC/25man raids - > 25man HC raids
MoP: Quests -> Dungeons -> HC Dungeons -> Raidfinder -> normal raids -> HC raids - Mythic Raids
WoD: Same as above
Legion: Quests -> Dungeons -> World Quests -> HC Dungeons -> Raidfinder -> Mythic+0 -> Normal raids -> Mythic+X -> HC raids -> Mythic+Y -> Mythic Raids/Mythic+Z
BFA: Same
Shadowlands: Same
There have basicly been added more and more levels of difficulty and rewards, making the previous tiers basicly irrellevant. Added on to that, the fact that you can basicly keep grinding mythics forever in hopes of the best drops in the game, when paired with the player mentality of having to min/max everything, kills the enjoyment for a lot of people. Sure, some people like it, but a lot of people miss the time where they could have a list of items to aim for, and feel they achieved it when they get one of the pieces of loot they were after.
The removal of titanforging helped the issue a bit, but the core issue is still mythic+.
Well, that's at least my - somewhat incoherent - view on the matter.