GoV is an excellent source of skill-up, so is leveling NPC, so is making an elemental Empy weapon, and if you are truly daring you can do all at once!
As to campaign skill-ups, I agree with the OP. I've never seen 1 single, justifiable reason to NOT allow skill-up in campaign since the cap increase to 80; pre-80 it was to keep non-campaign content active and vital. However, SE specifically removed the little bit of skill-up that was available from campaign about 2 years ago iirc. In the almost 9 years since SE started the game they have never repealed a change they have made, they simply tweak other content until they get back approximately to where they started and the process can take years to accomplish. As an example, when they nerfed Beast to address a non-existent issue regarding MPK it took almost 5 years to get beast back to a point where it was on par with how the job was designed... and then they removed charm from any relevant content. No, I think it's safe to say that skill-up is gone from campaign for good.