That honestly always felt like BS. Regardless, the animation has come a long way since then so they couldn't use that excuse anymore.
There are so many scenarios they could write to trade evo Boldore. Trade Festivals, Go catches a Gurdurr (or any other trade evo pokemon) and wants to evolve it, and Ash remembers Boldore, or Ash recalls Boldore for a battle, it loses, gets sad, has a desire to evolve, etc etc. It'd be pretty easy.
Please can none of Ash's Pokemon at the Lab evolve offscreen? That would be lazy as hell. Evolutions are always epic anime moments, showing the Pokemon overcoming struggles/ development or reward for hard work. Naganadal was a special case ( and still stupid and cheap).
Really don't get this argument that Ash can't get Kubfu because of type overlap. He's had type overlaps since DP. Also, Kubfu's whole thing is going to one of the battle towers and evolving into one of 2 forms. Evolve it into Single Strike Form and boom, you've got a Dark type. Plus we'll probably have Lucario by the time a potential Kubfu turns up (June, I guess)