I think we’ll need a fancier test than projection == null, per this comment on projectionAspectRatio:

A quick take, but should have more thought:

function hasProjection({projection} = {}) {
  if (projection == null) return false;
  if (typeof projection.stream === "function") return true;
  if (isObject(projection)) projection = projection.type;
  return projection != null;
}

But the bad part is that the projection can be specified as a function (a “projection initializer”) which can return null indicating that there isn’t actually a projection. But maybe we don’t care about that nuance here, since this is really just a performance optimization and shouldn’t change the behavior?

I don't think we even need to cover the projection: null and projection: {type: null} cases, tbh. Having a smart domain when projection===undefined would be enough to solve the casual use of Plot.geo with some geojson. If the user starts to specify a projection, then they know what they're doing.

Hmm it was the other way round: we can only skip that geometry look-up if the projection is guaranteed to be non-nullish. Which is what you were saying :)