Battle Royale is a custom gamemode developed by the Titan team for Team Fortress 2.
This gamemode took heavy inspiration from PLAYERUNKNOWN'S BATTLEGROUNDS which hit Steam on 23 March, 2017.
The idea of battle royale was developed many times by the team in the past but none of it came to get people interested in continuing to play it. Many did not know what battle royale is either at that time.
PLAYERUNKNOWN'S BATTLEGROUNDS then came out and 7 months later, it hit the most played game on Steam (beating Dota 2 which stood 1st place for eternity) in terms of the number of current players playing the game according to Steam Charts.
The team had to take the opportunity to reintroduce the Battle Royale concept to the Team Fortress 2 community now that this concept is a popular thing.
Unlike previous battle royale projects, this one was heavily inspired from PLAYERUNKNOWN'S BATTLEGROUNDS with mechanics like parachuting to the ground at the start of a match to finding the items you need with just a melee weapon.
Early Access 2 started on 8 October, 2017 which introduced the blue and white circles, the ability to loot dead player backpacks, a rework on the overall in-game user interface, a fix for items clipping through the ground, and hooking TAB to easily open the inventory.
The map used (ffa_hills) for this gamemode is available on the workshop which is rumored to be used for a free for all project which was sadly abandoned.