system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU/Thor Ragnarok] How did no one in Agard know who Hela was?

Asgardians are shown to be extremely long lived (living thousands of years) yet no one in Asgard had heard of Hela when she first arrived. Surely there are surviving Asgardians who remember her. Also if she was imprisoned before Thor was born, does that mean that Valkyrie is way older than Thor since she knew all about Hela?
gpt
While Asgardians are extraordinarily long-lived compared to humans, Odin was probably exceptionally long-lived for an Asgardian. His use of Odinsleep and the Odinforce probably allowed him to live multiple times the life of a normal Asgardian.

So to the general Asgardian populance, Odin's period of conquest was still generations ago, and he'd specifically wiped out any mention of Hela and that bloody period of their history.

As to Valkyrie - she's been in exile on Sakaar since facing down Hela, and time is... funny on Sakaar. While sometimes time passes faster there (Loki arriving weeks before Thor, despite only a few seconds passing in the Bifrost) and other times it passes at the same rate (Hela seems to be in Asgard for roughly the same period Thor is on Sakaar, give or take a couple days), on average time appears to pass more slowly there. Valkyrie has been gone from Asgard for thousands of years, but only a few centuries have passed from her perspective.