The Kartoza GIS gals and I were fantasizing about being able to make this effect when I realised the LineBurst style (only available from QGIS 3.24 onwards) would let us do it. We were working on some OSM styling and I was thinking that it might look nice to have the minor roads blend into the major roads. The idea was to create a bit of an 'onramp' feel between minor roads and major roads.