“Battle royale” is simply a recent development because up until recently we barely had any 3D shooters which had maps huge enough to allow for this style of play.
Particularly since the movie Battle Royale had a mechanic that filled the same role (sectors of the island were declared off limits to force students to encounter each other.) Not quite the same implementation, but I think basically the same idea.
Multiplayer Bomberman had this back in the 90's! I loved that game in high school.
But yeah, combining it with putting 50 people in a FPS deathmatch came much later. In video games a whole lot of innovation is combining a few ideas that already existed somewhere else. Well, not just in video games, really....
It's often surprising how long it can take for things to become mainstream. The first Battle Royale mod dates from 2013. And Planetside supported hundreds of players in a single match back in 2003.