The original message began with the assertion that the OP had run across a bug in dash, and gave two URLs, with no description of the bug or the impact it was having on their life.
I read one of the URLs, and the bug is rather obscure. It involves a second script embedded inside a here document inside the first script, with the second script being passed to an interpreter process on stdin. I'm not surprised that nobody knew about the bug for many years. So, having found one obscure bug in dash, the OP decided that the best solution is to change Debian's years-old /bin/sh policy. This ignores the fact that all shells, including bash, have *lots* of bugs in them. Switching /bin/sh to another shell would simply be trading one set of bugs for a different set. Given that Debian *originally* used bash as /bin/sh, and made a conscious decision to switch that default to dash several years ago, it would take an overwhelmingly strong reason to revert that change. "I found an obscure bug in dash that affects me and one other person" is probably not strong enough, especially when the bug has been fixed upstream (albeit not in a released version yet??). A more productive course of action would be to open a Debian bug report against dash, describe the issue and how it affects you, point to the upstream patch, and hope that a patched version of dash makes it into trixie.
