Valve keeping up with the trend of "worst kept secrets". You need an invite to join the alpha but since everyone who owns it can refer their friends, it spread very quickly.
I've been playing it the past few days and it's honestly very fun. Still a bit rough around the edges (especially in terms of balance) since it's in early access, but it has serious potential to be dota 2 levels of popular.
For the unaware, Deadlock is a 3rd person shooter MOBA. It feels like a mix of Dota and Overwatch/Team Fortress. Nobody is allowed to share footage or screenshots, but obviously with so many playing there's a ton of leaks out there.
If you don't get the mail, this is probably due to playtest schedule, they may give you access when the playtest is open, after that you can always play vs bots
