Debian 9 (stretch) is now beyond LTS support and on to ELTS support,
which means that the typical Debian infrastructure is no longer involved
and updates are no longer available from Debian. As a result, it's not
really appropriate for us to continue to build packages on stretch since
we can't be certain our dependencies or build environment are secure.
It is the case that some of the OSes that use Debian 9 packages, such as
some versions of Ubuntu, do still receive security updates, but since we
don't build specifically for those OSes, we'll need to drop support for
them as well. Of the OSes receiving mainline (non-extended) security
support, only Ubuntu bionic is affected.
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