The list of standard features needs more careful review.
In particular, exact-complex seems odd - it's more likely
that we want to test for any complex, and if we really want
to distinguish exactness why not inexact-complex?
I don't know of any Schemes that support exact complex numbers but not inexact. The normal cases are all types of complex numbers, or inexact-inexact complex numbers only (like SCM and some others -- I don't have access to my suite of Schemes today). So the exact-complex test is for the ability to do exact arithmetic on complex numbers. To test for the presence of complex numbers of any kind, test for (module (scheme complex)).
Okay, back home. Support for all types of complex numbers: MIT, Gambit, Mosh, STklos. (Also CLISP.)
Support for exact/exact and inexact/inexact complex numbers only (no mixed exactness): Racket, Chicken (with the numbers egg), Scheme48/scsh, Kawa, Chez, Vicare, Larceny, Ypsilon, IronScheme. (Also Armed Bear CL, CMU CL, Clozure CL, ECL, GNU CL, Steel Bank CL.)
Support for inexact/inexact complex numbers only: Gauche, Guile, SISC, Chibi, SCM, KSi, Scheme 7, UMB.
No support for complex numbers at all: Bigloo, Ikarus, SigScheme, RScheme, Elk, VX, Oaklisp, Owl Lisp.
Equivalent to #320.