Hi Enrico,
> in the Developers's reference[1] it says, in boldface, that security
> updates should be built with "urgency=high".
This is incorrect advice and I have idea where it came from. The urgency
is completely irrelevant for any security upload to LTS/oldstable/stable,
only for testing-security uploads when these were still a thing (maybe that's
how this trickled in).
Cheers,
Moritz