They are the new icons. Something like Windows replacing XP icons with Windows 10 ones. It is supposed to ship as default with Gnome. But I don't know about Ubuntu or Mint. They do customize a lot on theme side. But if you are going to use basic Gnome on your PC, these will be the icons that will get to you
If I elaborated too much, these are the new app icons in Linux. These icons will ship default with Gnome 3.32.
You can see more icons here : Icon Grid