Early access was announced on March 28th, during the Gearbox Main Theater Show at PAX East 2019! You can read more about this at: http://www.gearboxpublishing.com/risk-of-rain-2-early-access-available-now/
Join Risk of Rain 2’s early access on Steam here:
https://store.steampowered.com/app/632360/Risk_of_Rain_2/