I can make an SVG layer in Glyphs 2 by naming it “svg”, and it will persist when the file is opened and saved in Glyphs 3 with Glyphs 3 file format.
But how to make such layer in Glyphs 3 directly? When I name a layer “svg” there, no special handling seems to be applied. When I drag an SVG file into it, paths are imported instead of placing the SVG as an image.