Should a sourdough starter be left open/ajar after feeding?
It's actually a bit of an old wives' tale that the yeasts colonizing your starter are from the air. Almost all of the yeasts in a starter are from the grains used to make it. This is why starters are frequently started with either whole grain kernels or grains like rye that are left mostly intact when milled. All of the microorganisms on the outside of the grain kernel are then included in your starter.

As to leaving your jar open, I certainly wouldn't seal it up, especially if the jar is glass. The carbon dioxide produced by the starter could cause the jar to explode if it can't escape.