From the description

Do we continue to support all those formats, or even more? (PNG)

I think we could avoid PNG. In case of OOjs UI we generate PNGs and colored options using a grunt task. Most projects can adopt a similar workflow.

When providing PNGs or any other raster format we'll also have to provide multiple sizes. Are there any known use cases where raster files are required?

It would be great if we are able to get design and production ready SVGs.
By production ready, I mean SVGs simplified and formatted so they render properly when for example converted to xml for Android.

Also potentially more expect all icons to be contained within a standard bound (eg., 48x48px) to minimise anti-aliasing and for a consistency in sizes of various icons.
This point relates also to T139351: Rules for Wikimedia icon additions .

I think SVG files should be our single source of truth, and generate the needed formats by exporting from them (automatically if possible).

As @RHo comments, design SVGs can be the source, and production SVGs derived from those.

Based on this comparison, I'm not sure we want to use a font-based version, but I'm ok if we export automatically to any format that can be useful to someone else.