Hide System Fonts feature (make system fonts toggable)?

I was perusing the source code for KOReader a little bit and I stumbled on this feature: https://github.com/koreader/koreader/pull/5670

I'm not up-to-speed on github and branching enough to figure out if this feature was actually ever fully implemented. If so, what is the mechanism of enabling/disabling system fonts?

As mentioned in the comments for the feature, I'm also looking at a way to avoid having 15 pages of Noto fonts in every language imaginable... would be nice to have some sort of filter...

EDIT: It does look like this feature is in the official release: https://github.com/koreader/koreader...s/tag/v2019.12, but I guess I can't find the toggle switch to turn system fonts on and off... can someone help me find that?