Project Treble images are less "generic" than they sound - they vary by stock-OS version (Nougat, Oreo, Pie, Q/10), 32-vs.-64 bits, A-only vs. A+B boot.
That's still only one source code for all of them. (Nougat never had treble) the difference there between actually one single image and five is build time. There is no additional development needed.
And that still means that with five images you target several hundreds (probably thousands) devices. More than lineage will ever support
There are unofficial builds of this [1] but lineage doesn't want to endorse this approach, even though that's technically how they support all recent devices.
https://forum.xda-developers.com/project-treble/trebleenable...