It seems that all failing games eventually go the free-to-play route. Sadly, for as good as Battleborn may have actually been, it was a pretty big misstep for Gearbox Software. No matter what Randy Pitchford may have said (including talking about Battleborn themed porn), the game never took with the public, so this new "trial" only makes sense.
Launched today for PC and Xbox One (coming June 13 for PS4), the free download grants access to all available modes and maps of Battleborn, but limits you to a rotating cast of six characters. This is similar to how Heroes of the Storm or League of Legends works, just that those games didn't originally launch as a $60 game with a season pass.
You can unlock the rest of the cast by earning currency or purchasing "Platinum" currency with real money. If you owned the game before this shift, you'll be given "founder" status and a ton of extra nonsense.