I can only say with certainty that they don't work for Android as I haven't been able to test on anything else. I followed the instructions here and while that did get the custom font to work for pretty much everything (labels, entry boxes, etc) it wouldn't work for Oxyplot no matter what FontFamily property I set.

So then I tried adding it as an Asset to the android project but it was still ignored.

Eventually I created a fork and got it working (at least for Android) by looking up the font asset in DrawText in CanvasRenderContext.cs, but upon reading the contributing guidelines when making a pull request it said to create an issue first and wait for it to be assigned.