In Starcraft II there are some achievements that require you to use abilities that were patched out with balance updates. I am talking specifically about the achievements in the "Versus" group. But it might be the case that other achievements could be unreachable. For example there is an achievement where you need to kill a couple of units with the Raven Seeker Missle. Well, good luck with that because the seeker missle was nerfed to do 0 dmg in the last big balance patch. Is it somehow possible to get that achievement?
If you've purchased Wings of the Liberty and/or Heart of the Swarm, I think you can still play these expansions. This way you get those abilities back and will be able to complete the achievements.
However, I'm not sure if that will allow you to complete every achievement in SC2.
This option was removed from the game, however you still can play expansions in custom games (Custom → Melee). Not sure if the achievements are granted in this mode.