In Thor: Ragnarok, Thor doesn’t seem to have heard of his sister, Hela. He doesn’t know what Odin is talking about when he mentions her.
Thor: Father, who are you talking about?
And he doesn’t seem to know her when they meet:
Thor: You must be Hela. I’m Thor, son of Odin.
Now, Hela is Odin’s firstborn child, so naturally Thor was born after her, and presumably after she was imprisoned. Further, Odin covered up a lot of the monuments and artwork showing her.
Valkyrie is aware of Hela (obviously), having fought her when Hela decided that she wanted to be in charge. Obviously this means that Valkyrie is older than Thor, but is she so old that no one else in Asgard was around to remember Hela? And if not, wouldn’t Thor have heard about Hela from one of them? Hela only seems to be a generation or so removed from Thor, after all, who’s not really portrayed as an extremely old Asgardian. There must be lots of people from Hela’s time.
On the other hand, other people don’t seem to recognize Hela when she returns:
Hogun: Whoever you are, whatever you’ve done, surrender now!
We know that Norse mythology is pretty similar in the MCU, and that it’s inspired, in-universe, by the visits of Asgardians to Earth. Hela, or Hel, is part of that mythology, and I’m pretty sure we’ve seen that Thor knows what humans think about him and Asgard in general. Did Thor not hear about her?
So did Thor really not know about Hela? And if so, was there just a big conspiracy to never mention her again, so that all the younger Asgardians (e.g. Hogun), including Thor, didn’t know about her? Some spell cast by Odin?