With the removal of infested terrans and several other changes from recently, some achievements are easier or harder now. In this case, the infested terran achievement requires killing 30 units via IT spell, which doesn’t exist. Unlike other achievements as well, this can only be earned by going into unranked or ranked.
A remedy for this of course would be to switch expansion; however, many new players do not have this knowledge, and thus Blizzard should review current achievements and remove/edit to match the current playstyles.
Is this important? No, but it’s housekeeping that is much easier to implement than balance.
Is this necessary for gameplay? No, but players who are more casual such as myself strive for these small things like goals or having fun (when possible).
Overall, Blizzard needs to not only work on developing a better meta for the game, but should review their structured set up in terms of achievements and non-ranked play for a more well-rounded and up to date experience.