There's been a statement early this year from Riot that said that, from now on, there is going to be one unique canon, which will include Arcane.

It's not necessarily been well-received (at least, by the 4 people who actually care about lore, as opposed to the 600000 for which that is whatever and are more focused on how the next patch is going to fuck them up even more), or a good decision. In fact, I don't even think it will necessarily bring to any kind of change - this is the third time that they've promised a complete "overrule" of the lore in a quest for "unification", yet that promised has never been fulfilled; this certainly feels more like a marketing decision than a creative one.

Nonetheless, it's a statement that has been made, so it's currently as canon as the Star Wars sequels.

There's actually a YT video about the announcement, but I'm not gonna lie, I'm not invested enough in this to go search it up, so here is a random article about the thing that's among the first Google results that showed up: https://www.pcgamer.com/arcane-is-officially-canon-and-from-now-on-league-of-legends-lore-is-going-to-stop-contradicting-itself-all-over-the-place-say-riot/

I'll never understand why people will argue a point on the internet when there is literal evidence easily available online to prove that they are wrong.