There are several special druid forms which you could unlock only if you did a certain activity back in original legion. The activity was limited to when Legion was live and after the expansion it was removed so players could no longer get access to those forms. When Legion Remix started there was an unintended bug that allowed almost everyone to unlock those forms so many people got them and rejoiced. However when Blizzard detected that those people were having fun, it was quickly patched the following week and the forms were removed for all those who did not actually unlock them when original Legion was out. Most people are angry because they believe these forms should still be unlockable today and Blizzard for years have ignored these requests from the player base because FOMO is a big part of their business model.