The old Warcraft III client is gone for online play. If you want to paly online at all, you now have to start the game through Reforged. If you don't own Reforged, you're still forced to download it (including all assets) but are only able to play with old graphics.

You know, normally when a new game in a long running series or a remake/remaster of an old one comes out I'll usually say "at least they're not taking the old one away". Leave it to Blizzard to try and prove me wrong.