Mapbox supports the popular Web Mercator
projection, and currently does not support any other projections as output.
Web Mercator is a nearly conformal projection that is adopted by the vast majority
of web maps and its use allows you to combine Mapbox’s maps with other
layers in the same projection.
Commonly this projection is referred to as
EPSG:3857: see epsg.io for more
information and alternative encodings.