The following guidelines are not required for submission to Flathub, but are best practices we recommend and consider for curation and promotion. These guidelines are designed to help your app and Flathub itself appear in the best light to users—they may change over time based on developer and user feedback.
I do think that as a store it should have some guidelines on quality, but not of the application icon’s styling
Some of the icons they say are good aren’t flat though, they feature 3D shading and clearly show depth.
Take that GNOME-y looking one for example. It looks modern, but the workbench, screwdriver, spanner, and hammer all have shading and depth. They are 3D objects. The genie-lamp icon looks far flatter, and that was disapproved of.
Flat and modern aren’t really the same thing. There are old flat designs, and plenty of modern icon designs are slowly becoming more 3D again.
Some of the icons they say are good aren’t flat though, they feature 3D shading and clearly show depth.
Take that GNOME-y looking one for example. It looks modern, but the workbench, screwdriver, spanner, and hammer all have shading and depth. They are 3D objects. The genie-lamp icon looks far flatter, and that was disapproved of.
Flat and modern aren’t really the same thing. There are old flat designs, and plenty of modern icon designs are slowly becoming more 3D again.