Hi,
Buster has 0.7.90 of console-common, which unfortunately has a bug so that the install-keymap binary was accidentally excluded from the package.
It was fixed early September in 0.7.91 and migrated to testing 2019-09-13:
Sadly, this fix has not yet made it into stable. Is something holding this up, or has it just been forgotten?
I'm working to update a custom "edition" based on something like old-old-old-stable to Buster and am now stuck in this because I cannot get the boot keymap I want.
Kind regards,
Oskar