In Clojure (and I’m pretty sure Scheme), ! and ? are not sigils in the sense that they’re special syntax like, say, @ (which is a reader macro), or @ and @@ in Ruby: they just tend to be used by convention. ! and ? are no different than a or b.
They somewhat follow the OP's definition of sigil (in the yellow box), which doesn't say that sigils have to be interpreted in a special way, only that they convey some meaning to the programmer (although it's a bit unclear - communicate to whom, programmer or interpreter?). They are also not at the beginning of the word, but that seems to be rather arbitrary requirement.
I perceive FORTH in a different way. The only sigil in FORTH is SPACE. Everything else is a letter.
Then you have Bjarne Stroustrup's "Generalized Overloading for C++2000", with which you can overload whitespace, uniquely redefining the meaning of space, newline, tab, and even the absence of space.
You can read more about the Clojure reader here: https://clojure.org/reference/reader