https://www.cnet.com/news/world-of-warcraft-classic-launches-in-summer-2019/

No, that's not as soon as some players were hoping, but fans won't have to wait until summer to get a taste: Blizzcon attendees will be able to play a demo of WOW Classic at the event. The demo will also be made available to digital ticket holders through Blizzard's Battle.net app. 

Unfortunately, the demo isn't exactly an early beta of the MMO -- players will be limited to 60-minute play sessions with a 90-minute cool-down. There's no limit to how many times you can play it, but you can only play it for an hour at a time. That's not enough for a marathon MMO session, but it's enough to help you figure out if you want to return to the Azeroth of old.