Let's see what's been documented for 10+ years (see commit below) on
https://www.debian.org/releases/trixie/amd64/ch05s03.en.html#module-parms
5.3.1. Boot serial console
[…]
Be sure to specify this option after “---”, so that it is copied into
the bootloader configuration for the installed system (if supported by
the installer for the bootloader).
(Arguably it could be moved elsewhere within that section to make it
easier to spot.)
Commit: https://salsa.debian.org/installer-team/installation-guide/-/commit/615fcaa11aa37bfd5b6295d02a9e642e5d2e2f48
Cheers,