In Dutch, ij and ÿ are interchangeable.
My Dutch professor in college actually wrote out the "ij", but the tail of the "i" connected with the "j", making it appear to be "ÿ". It kind of blew my mind the first time I saw him write it out haha
In cursive, it is often indistinguishable too.
In print, NO. You really can't interchange the two. As already said, in handwriting they may look similar, however we would never mean the actual ÿ, since it simply is something else which is used in Greek and French (and others).
There's even a section about this on wikipedia: https://en.m.wikipedia.org/wiki/IJ_(digraph) subsection "technical details".
At the very least it happens in Flanders. I've seen ijs ("ice") capitalized in cursive as Ys[0] on an ice truck, for example.
Edit: from Wikipedia[1]: "It used to be common, in particular when writing in capitals, to write Y instead of IJ."
So it's an obsolete practice...
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0. something along those lines: http://alphabetprintables.org/alphabet_printables_cursive/up...
I think you mean Dijkstra, who did say that, because of an argument with mathematicians about indexing from 1.