I'm not sure whether it's a Belgian or Dutch thing and what the context is, but I've seen it written as something very similar to ÿ (both in handwriting and in

I'm not sure whether it's a Belgian or Dutch thing and what the context is, but I've seen it written as something very similar to ÿ (both in handwriting and in print) and I've also seen variants of a broken Ü as uppercase ligature. I'm not saying it's particularly widespread or widely accepted, but I've seen it in real-world use by native speakers. For all I know it was just a stylistic choice or very experimental, but the same could be said about German uppercase SZ (ẞ -- vs the lowercase ß) and that one even has its own Unicode codepoint (although most people pretend it doesn't exist). It doesn't seem too surprising either. German umlauts for example evolved a lot throughout fairly recent history (by European standards) -- we used to have a tiny superscript "e" instead of the two dots. EDIT: To clarify: I was apparently wrong about the uppercase version (doesn't seem to have dots) but the case for ÿ seems pretty solid.