Lots of systems are dealing with this issue, it seems, judging from
[1] https://en.wikipedia.org/wiki/Bottom_type

I don't think it's a bottom type -- *unspecified* _is_ a value, so if we assign a type to it, it is inhabited, and hence not a bottom type. I

What about using characters from some private upper unicode section
to represent various kinds of unspecified things? E.g.,
as guile named chars,

#\unspecified_function_retval
#\unspecified_function_error
#\unspecified_macro_err
#\unspecified_exception

I think it would be better to:

• gradually move away from *unspecified* to (values)
• and this way, gradually change the meaning of *unspecified* from "an unspecified value" to 'an atom you can do with as you want"
• after this, unspecified? and making #<unspecified> readable by the reader aren't weird anymore

though more something for guile-devel@ I suppose.

Greetings,
Maxime