It’s a kickstarter game, so I think paid DLC would be kind of a slap in the face of their community (in my opinion).
They could give backers free DLC codes - pretty sure that’s what the Hollow Knight devs did for their backers. Heck, I’m pretty sure everyone who backed the HK Kickstarter is getting the sequel for free too because Silksong was originally pitched as stretch goal DLC for the first game.