(I got it from a Dutch tech news site for source reference: Microsoft Store verbiedt betaalde apps met opensourcesoftware van derden - Computer - Nieuws - Tweakers)
In cases where you determine the pricing for your product or in-app purchases, all pricing, including sales or discounting, for your digital products or services must:
- Not attempt to profit from open-source or other software that is otherwise generally available for free, nor be priced irrationally high relative to the features and functionality provided by your product.
So people selling Darktable (and other FOSS projects) without any changes and just trying to trick users in paying for it, is now officially not allowed by Microsoft anymore.
How you’re going to police it, is another question to be honest… and it probably requires things like ‘proof you made changes to it, so fork it’. ‘Proof that it is yours, so you may sell it’, etc…