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.
They just launched some new Pathfinder books that are effectively pathfinder 2.x, with a lot of small (and some larger) tweaks, but technically the previous 2e books are still able to be used interchangeably.
To that end, there’s a Humble Bundle going on where you can pick up a TON of that legacy 2e content in official PDF form, so if you’re interested you should check it out! I believe most tiers include the Beginner Box, which has an intro adventure for new players and includes some sort of single player content that would give you a glimpse into how the game runs :)