I've come to realise that we could even generalise on the step adjustment by allowing arbitrary connections between points.

one consideration/complication for ribbons is how to close the last part of ribbon especially if it is a CI around say a geom_stop of a kaplan meier curves:
https://adibender.github.io/pammtools/reference/geom_stepribbon.html
https://github.com/smouksassi/ggquickeda/blob/main/R/ggkm.R#L750

Thanks for the links @smouksassi! I have the feeling like I'm missing some subtlety here. Do KMCs have some special rules about how the steps should be drawn?

I think {ggsurvfit}, which I trust on their KMCs, uses direction = "hv" by default (https://github.com/pharmaverse/ggsurvfit/blob/main/R/stat_stepribbon.R).

harrelfe/rms#70
I will have to prepare something specific to KM, before deciding geom_kmbands I had to review what is available and what was different between the various solutions. from what I remember it is mainly the last point and how to close it that might be different between "methods"

If we now already have 4 examples of packages had to reimplement the steps, then I suppose that indicates some need to support this at the base ggplot2 level beyond geom_step()😅