Coordinate systems and projections are one of those deeply complicated truths that makes such a headache in GIS. I still shudder at all the pain in school and at previous jobs dealing with inconsistent datasets.

It seems like they decided to just opt out of trying (see the yellow box in section 4): https://stevage.github.io/geojson-spec/#section-4

I think they should have completely backed off from touching on projections and datums in the format altogether. Ie. Something like, “coordinates are 2 or 3 tuples where the values in order correspond with easting/long and northing/lat and elevation/altitude. See metadata for agreed upon units and CRS/projection semantics. It is strongly encouraged to standardize on WGS84 when encoding data with an earth-resolvable datum.”

Because GeoJSON otherwise works fine for indoor spaces, video game spaces, fictional lands, other celestial bodies, etc. You just have to educate on the idea that there’s more to data compatibility than it being GeoJSON.