At the end of Thor: Ragnarok, after Thor becomes Electric Thor, there is a short scene where Thor tells Loki that Surtur's Crown/Eternal Flame is needed. Within 2-3 minutes, the scene after of Thor and Valkyrie vs Hela, Hela impales Thor with one of her spears. I re-watched it, and you can see it goes in one end and out the other.
What happened to his arm there or was it just forgotten about completely?
Whilst it does seem to have damaged him it was mostly a surface wound. Thor's body is turning with the spear so it ends up running almost parallel with him rather than straight into his body which minimises its effect.
When Hela then pulls on the spear and flings him away we get a clear look at his back where the spear should have come out and we don't see it so it didn't go through.
If you watch the scene below, starting at 1:10, you see the spear falls out/off of him when he's flying through the air and then completely when he bounces off the Rainbow Bridge.
Later we get a close up of Thor as Surtur brings about Ragnarok, the area of his armour on his chest where the spear went in his clearly damaged.
It's also worth noting that Thor has an incredibly high regenerative healing factor; much more so than other Asgardians. What little damage the spear actually did to him probably didn't bother him for too long.
