Only quoting Humankind's Twitter account here :

"There won't be a traditional early access period—we chose to try a new way to gather the community's feedback with OpenDev. However, there will likely be more opportunities to try the game out before release next April!" https://twitter.com/humankindgame/status/1326189778769948673

"We haven't announced anything just yet, but there will definitely be more opportunities to try Humankind before release!". https://twitter.com/humankindgame/status/1323997029790588928

Now, when will this/these new opportunity(ies) come, that's the big question of course !

Since both previous OpenDev were available during holidays (Summer and All Saints Day/Holidays), one could assume there could be one for Christmas Holidays. Or next year's Winter Holidays (between February and March 2021). Though, since the game's launch is due on April, I doubt on the latter, it would be too close to the game's launch.

Time will tell I guess !

stadia is ok but doesn't do too well when you start going up in population and teritory.  open dev on steam is preferable

also,  tabing out and then back in is a sure fire way to crash your game after a couple hours