One loss due to this change is the ability to represent an invalid UUID (vs a NIL UUID).

There is actually no meaningful distinction between a hostname which happens to be a second-level domain and a hostname which has more parts than that. You can even have a machine at a top-level domain if you happen to be a small country or otherwise are able to create an A record for one.

When I speak of Europeans or mental Europeans, I'm not allowing for false distinctions. I'm not saying that on the one hand there are the by-products of a few thousand years of genocidal, reactionary, European intellectual development which is bad; and on the other hand there is some new revolutionary intellectual development which is good. I'm referring here to the so-called theories of Marxism and anarchism and "leftism" in general. I don't believe these theories can be separated from the rest of the of the European intellectual tradition. It's really just the same old song.

While there are many great answers regarding the "glyph not found" glyph, that won't help you actually detect it, as the text string in code will still have the character regardless of the font used to render it.

They reduce the distinction between a “core module” and a “plugin”

I think that over time the distinction is lost. My math teacher, 35 years ago stated "formulas are used in chemistry, in math we have equations". To this day, the word 'formula' in math seems wrong, but I'd accept it's used commonly.