The new version FAI is available in two variants. FAI 5.8.4 is for Debian buster and FAI 5.8.4~bpo9+2 is the same for the stable distribution called stretch, including the configs for stretch.
You can get the packages when adding one of these lines to your sources.list:
deb https://fai-project.org/download stretch koeln
or
deb https://fai-project.org/download buster koeln
New FAI ISO images using stretch are now available from [1]. The FAIme build service [2] for customized cloud and installation images also uses the newest FAI versions.
[1] https://fai-project.org/fai-cd/