I can only assume it's some unimplemented feature.
I'm not exactly sure what showing constellations would do on a 3D map. The stick-figures would be so distorted as to be useless. You can turn stick-figures on in Celestia and they just look like a mess of long lines radiating out from Sol (not too dissimilar from your avatar actually). Each constellation could also correspond to a long, thing wedge of space radiating outwards from Sol, but again, but sure how you would display these on the galactic map in any meaningful way.
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