In general D&D has flavor-gates in that the usual progression is to start out reasonably close to real-world European medieval reality and move on to more fantastical elements slowly. For example, Fly is traditionally flavor locked to start appearing around 5th level when you get access to the 3rd level spell Fly and uncommon magic items like Flying Brooms start appearing.
I tend to feel that's fine for Greyhawk and Dragonlance. It would also have been fine for the Faerun of about 100 to 300 years ago. But Faerun as it currently stands has experienced a lot more in the way of cosmological events. Note that the events of Descent to Avernus have recently happened not to far away. A little bit before that the Temple of Elemental Evil would have been kicking up a storm a bit further north, and this is also after the Tomb of Annihilation events... among other things.
Also note that Out of the Abyss started with the party kidnapped by drow and escaping during a battle between demons and the drow.
That said. Temple of Elemental Evil does start with small events like bandits and stirges and leads up toward apocalyptic cults summoning eldritch abominations.
So, starting with weird and traditionally dangerous monsters is far from unheard of.
