Proprietary packages may still be reproducible. Presumably, you verify a hash of the inputs (e.g. upstream tarball/binary and patches), and the output should still be bit-for-bit the same on any machine given the same inputs.
Not to mention that there are many open-source (as in OSI) packages that may not be distributed with Guix System because they use non-FSF-approved software licenses.
> Not to mention that there are many open-source (as in OSI) packages that may not be distributed with Guix System because they use non-FSF-approved software licenses.
This is not true; at the very least "many" is wrong in the above statement. Generally, there is virtually no difference between the OSI set of approved licenses and the FSF set.
Can you list examples of the many packages that would be considered "open source" (according to the OSI definition) but not also Free Software as per the FSF definition?
Not to mention that there are many open-source (as in OSI) packages that may not be distributed with Guix System because they use non-FSF-approved software licenses.