Setting keyboard-configuration/xkb-keymap seems not enough for Debian 11.6.0. Using the netinstaller, I still got asked for my keyboard layout, after downloading the packages. It could be fixed by also setting keyboard-configuration/variant as workaround:
d-i console-setup/ask_detect boolean false
d-i keyboard-configuration/variant select Deutschland
d-i keyboard-configuration/xkb-keymap select de
What doesn't work
I also tried some combinations (each paragraph is one try) with different variants by setting attributes like the layoutcode or variantcode, but none of them worked:
d-i keyboard-configuration/xkb-keymap select de
d-i keyboard-configuration/xkb-keymap select German
d-i keyboard-configuration/modelcode string pc105
d-i keyboard-configuration/layoutcode string de
d-i keyboard-configuration/xkb-keymap select de
d-i keyboard-configuration/layoutcode string de
d-i keyboard-configuration/variantcode string
Additionally, the following were set for all of those combinations:
d-i console-setup/ask_detect boolean false
d-i debian-installer/locale string de_DE
It seems that console-setup/ask_detect doesn't had any effect. This is just for reference purpose, in case anyone would like to try this out. At least for Debian 11.6.0 netinstall, they didn't work.