Horizon was removed almost two years ago, in Bokeh 0.12.7. Just as you suggest, the reason is that there was no one to maintain a very high-level API in the core Bokeh project, and other outside projects (e.g. Holoviews or Chartify) were interested and better-suited to focus on this role.
I think a Horizon chart would be a reasonable thing for Holoviews to consider, and I think the Holoviews devs would be receptive to discussing it. I’d suggest proposing this feature on the Holoviews tracker. In the mean time, Bokeh 1.2 did add new
vline_stack methods to figures. As long as your “index” (i.e. x-axis coordinates) are aligned, then this should make it substantially easier to implement a Horizon chart yourself in pure Bokeh. You can see a
varea_stack example here.