Silksong contains remnants of all sorts of cut content, unfinished fragments of a game that never was. One of the things left out was an alternate version of the Last Judge fight that would've appeared in the game's secret third act. YouTube user Kilroy Was Here tracked down the snippets of leftover code for the fight, modded them in, and immediately realized why Team Cherry decided it should probably stay buried in the game's files.
Even though many players fought The Last Judge at the end of act 1, she's technically an optional fight. There's another way to get to act 2 that involves fighting a completely different boss, which leaves The Last Judge alive for whenever you decide to go back. However, you can't leave her alive if you want to enter the game's optional third act as she prevents you from completing one of the requirements to access it.
