When a new expansion comes out for World of Warcraft, some things have to be torn down to make room for the new stuff. This has been the case in the past and will continue to be the case as the transition into Battle for Azeroth is underway. Here’s a list of some of the things that will be disappearing.
Blizzard isn’t being totally cold and indifferent about it; they know that some people have been working towards certain goals for a while and it wouldn't be fair to end their quest without fair warning. So what they are doing is putting systems in place to expedite the rewards. As an example, a kind of silly and contrived quest will help powerup a player’s artifact weapon, which will then continue to morph at some point after the expansion.
There’s no word yet on exactly how much time players have left to complete certain bits of content, because there is no official release for the pre-patch that will change everything. But with the expansion itself only 47 days away, according to a counter on that aforementioned post, it is fair to assume that there isn’t much time left at all.