Should this also make explicit which Debian suites have this restriction?
I thought this rule also applied to backports having found [0] in a list
archive search, and hence have been explicitly changing dependencies for
backports [1] instead of using alternatives.
However after finding this proposal, I checked build logs [2], which
suggest that sid (including -ports architectures) and stable do but
backports doesn't. (Though we should probably check that with someone
who knows this better before writing it into Policy...)
[0] https://lists.debian.org/debian-backports/2011/08/msg00089.html
[1]
https://anonscm.debian.org/cgit/pkg-opencl/beignet.git/commit/?h=jessie-backports&id=b1cf2fe7c0d392cc4f99e31458e871de1fe2a574
[2] comparing "Merged Build-Depends" and "Filtered Build-Depends";
https://buildd.debian.org/status/logs.php?pkg=linux is a suitable example