I mean, I will argue that following the expansions allows for a universally coherent(?) or at least universally accepted thread for everyone to connect to in walk-up roleplay, but I also will argue that just because there was these world-ending events running a pain-train on Azeroth and her inhabitants doesn’t mean everyone was involved in every single event.
I would argue that, yes, a lot of people would know about the big, world-shaking events from a second- or third-hand perspective, be it adventures talking about what they’ve seen and where they’ve been, or veterans talking about the conflicts they served in, but actual ‘Story-Of-The-Expansion’ stuff?
That’s likely a group of less than a few hundred people, at best, who have been running around smiting Dragon Aspects and Old Gods and probably don’t go down to the bar trying to sway the swains with their heroic struggle against greased-up tentacle-monsters.
I do think it is important to have the expansions acknowledged, at least insofar as they happened.
The average adventurer/mercenary has likely heard of these things, but may have only ever engaged with the fringes of the ‘story’ of the Expansion. And players who want to say “Oh, no, I was in the thick of it. This happened.” are also justified with this method.
I do think it is very on-the-nose to claim you were the hero who kicked Argus in the cosmic conjunction or similar events, but I will argue that acknowledging that the events of the Expansion happened, and not outright dismissing other people’s stories, is important.
Sargeras showed up, large enough to rival the size of the Planet, and shoved Gorribal into Azeroth hard enough to wipe out Silithus, which likely caused earth-quakes and tsunamis around the world. He would have been easily visible to everybody in Kalimdor.
The floating fortress and symbol of the Twilight Hammer is visible from Ironforge mountain and Dun Morogh, as is the shattered dam that once kept the Wetlands from being completely flooded. The Barrens is still splint in twain by Deathwing’s efforts and Twilight Hammer Cults actively draw in thousands of Men, Elves and Dwarves in the Eastern Kingdoms, and it is unlikely all those people all died at the hands of the ‘Champion’.
Deathwing literally raided Stormwind, knocked down the statues, destroyed the Park District and melted parts of the front gate of Stormwind, and his blood is known to cause direct infection of Old God whispers, as well as being so toxic that wounds inflicted by it will not heal, if they don’t kill you outright. Even Alexstraza, one of the most potent and skilled wielders of Life Magic, is still scarred to this day by her brief tussle with the former Earth Warden on slopes of the Vermillion Redoubt.
Almost every Expansion has had massive incidents or accidents where large swathes of the civilian population has been exposed to cosmic horrors or cataclysmic battles with the Expansion Big Bad, or their servants.