Ballot 8 was conducted informally on a poll thread of the WG1 mailing list.
The original proposal was to make (string-set! str n #\null) unspecified, which was rejected, at least partly on the ground that R7RS implementations can already forbid specified characters from appearing in strings. However, the ASCII character set (which includes #\null) could not be forbidden, per UnicodeCowan. Consequently, the rationale given by several voters that there was no need to make a special case for #\null was incorrect. The WG therefore chose from the following options:
forbid-nothing: every character supported by the implementation must be allowed to appear in strings.
forbid-any-but-128: every ASCII character including #\null must be allowed in strings.
forbid-any-but-127: every ASCII character excluding #\null must be allowed in strings.
forbid-any-but-97: every printable ASCII character plus #\space, #\tab, and #\newline must be allowed in strings.
open: any character may be forbidden in strings.
reader: the characters required by the reader, plus [A-Za-z], must be allowed in strings.
unspecified: say nothing.