In trying to combat Fortnite's rising popularity, PUBG Corp has been adding limited time gamemodes to PUBG. A few weeks ago, a 50-player deathmatch mode called "Desert Knights" went live, but was quickly removed. The mode had been causing server issues for every other aspect of the game, so it was cut to fix those. It is now back for the weekend and should be already available if you've updated your game.
The mode, itself, takes place on Miramar (the desert map) and features a much smaller blue circle that never changes. 10 teams of five people will drop onto the map and murder each other until one team scores 200 points. Kills grant three points and knockouts grant one, though killing your own teammates will deduct five from your total. Revives are dramatically shorter, so you won't need to sit around waiting to get back into the action.
This sounds similar to the "War" mode that was added a few months back. That saw 30 players on 10-man teams doing much the same thing as "Desert Knights." I did get to try it and I am not a fan of having to drop from the plane after every death. That being said, I do believe PUBG could facilitate other game modes if they were tweaked better.
In any case, you'll have until the end of the weekend to try out this 50-player madness.