It took quite a while but Awoo now finally has a vector favicon for Safari pinned tabs! Since it has to be a monochrome icon I decided to go with the dark background color, which hopefully has enough contrast in both dark and light themes, and looks very nice on the touchbar as well if you're viewing your favorites
To make it load you have to unpin the tab, close Safari, delete the icon from ~/Library/Safari/Template Icons, relaunch Safari and reload the page (Alt+Command+R)
@noiob rather specific requirement on Apple's part for favicons to work huh? looks good nonetheless
@shaderphantom in the example pictures Google uses a Bitmap favicon