What are the default units of elevation/altitude/z in GeoJSON?

The current correct answer (as mentioned by John Wiseman in a comment on the question) is:

An OPTIONAL third-position element SHALL be the height in meters above or below the WGS 84 reference ellipsoid.

This can be found under "4. Coordinate Reference System" in RFC 7946 available at https://datatracker.ietf.org/doc/rfc7946/?include_text=1

Meters above mean sea level, per the wiki specification doc:

WGS84 is implied and coordinates represent decimal degrees ordered as "longitude, latitude [,elevation]" with z expressed as metres above mean sea level per WGS84.