> Debian maintainer (and upstream dev) is open to an upstream patch to add such an option

Wild interpretation right here.

There are only 2 realistic choices: Leave it as is or patch out the warning message in the Debian package

Debian maintainer is clearly deflecting the responsibility here because everyone knows very well that upstream wouldn't accept such a patch.

As it's already explained in the bug report, since Debian has no plan to do that migration in the near future, aforementioned warning isn't only useless and annoying, it's also potentially harmful, thus the correct action would be to remove it downstream like they did it in xscreensaver (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=819703#84)

But then they'd face the wrath of Lennart, so the only choice left is ignoring the report.