When do bosses on kite shields disappear? Am I correct in seeing that they appear to be smaller and more decorative than on the round Viking or Saxon shield?

In the 11th c, bosses on kite shields are fairly common, but by no mens universal. Perhaps as a transitional phenomenon.
Shields with bosses do show up ocoationally in the following centuries. Some are small and probably decorative, like you mention, and disappear sometime in the 12th c.
Others appear to be functional. Even heater shields with bosses are shown once in a while, often in combination with iron fittings on the front. This might be down to personal taste. I have not found much use for it, though.