The specification of the procedure procedure-arity-includes? seems to
imply the presence of arity information for all procedures.  In
particular, the null implementation seems to imply that applying any
procedure to any set of arguments will always result in an arity
error.

I suggest changing the specification of procedure-arity-includes?
to say:

Returns #t if the proc may be able to accept k arguments and #f
otherwise. If this procedure returns #f, applying proc to k arguments
will result in an arity error. If no arity information is available
for proc, returns #t.

(the last sentence above is an optional clarification only, that may
not be worth including the SRFI document)

and changing the null implementation to always return #t.

Cheers,
~Alexey