Nerfs tend to come a good while after heroes are released. They take their time to be absolutely sure a nerf is necessary, and then they carefully approach how the nerf should be done. They get a lot of flak from their players, but they are not hasty people.
Post-release nerfs are given rarely and reluctantly. Nerfs come for some heroes that dominate the meta so hard that even the old “best of the best” just can’t seem to match them.
I was here through the whole Telluria/Vela fiasco. Frigg/Odin are not nearly as devastating to the player base as Telluria & Vela used to be, but I expect that’s because it’s far easier to draw a monthly than one out of 16 Valhalla heroes. Telluria was everywhere at the top when she came out, but it’s much harder to draw Frigg or Odin.
I’m not sure whether they’ll follow the same pattern here as they did with Telluria/Vela. But back then, they put nerfs in beta and took a little time testing a number of different nerf tactics. I doubt they’ll go that far for Frigg & Odin. I’ve fought Frigg & Odin and I personally believe their damage output is in dire need of a decrease (and not their defense & mana effects). However, Frigg & Odin aren’t going to get as much attention because they’re a little easier to work against and they’re much harder to find.
Here’s a history of buffs & nerfs if you want it: