Warcraft 3 Classic is no longer available for purchase via the Blizzard Store. Trying do to so gets you a simple 'can't purchase' page which tells you that Warcraft 3 is not available, and to stay tuned for more information down the line.
Keen readers will remember that, aside from the whole Diablo Immortal mess, there were a few other announcements at Blizzcon 2018 as well, one of which was the confirmation of Warcraft 3: Reforged, a remastered version of the original.
Though Warcraft 3 Classic is unavailable, Blizzard did announce that all of those who purchase Warcraft 3: Reforged would get a copy of the Classic version as well, which plays a part in the upcoming remastered version's general appeal.
It could be argued that getting rid of the option to purchase the old original is a bit worrying, since it is very likely that it won't be available outside of the Reforged purchase anymore, but then again, the game is pretty old, and whoever wishes to get into it now will almost certainly be more interested in Reforged anyway.
Warcraft 3: Reforged promises a vastly improved graphical engine, more content, a virtually limitless moddability, and lots more - all to bring the legendary title up to par with the latest gaming sensibilities, though without getting rid of what made the game special in the first place.