I've decided to run the Kingmaker AP for PF2e and purchased the module to run it on Foundry. The issue I've encountered however is, Foundry recently updated to version 13, and now the module doesn't work anymore. It's unsupported as of this version.
Is the module being actively worked on to bring it in line with the new version? Or, am I stuck downgrading my Foundry version for as long as I want to run Kingmaker?
Ive been running into this exact same issue. Im still fairly new to foundry, only been using it for a year, so I didnt think anything of it when I did so. But it isn't just Kingmaker. Abomination Vaults is also totally broken. I assume some of the others are as well. Of the 3 I've purchased, ONLY the Harrow deck works. I hope they go over ALL their modules ASAP
PF2E on Foundry V13 is still in Beta, so you are probably better off rolling back to V12 until the basic system is set up. Most developers (including Paizo) will update their premium stuff to V13 soon.
A general rule of thumb with Foundry is to be patient and cautious with updates, particularly with major ones, and make sure any mods you have been running (like Workbench, Toolbelt, etc.) have been updated to be compatible with the new version as well.