Assuming not much has changed I can see a few differences:
- The UNSC (known as the Union of Socialist Star Systems or USSS) would have the military either disbanded or defunded heavily, either way the covenant war would be force a rearmament. I imagine the war would be as disastrous as the Gold Timeline albeit a little less
- The Sangheili and other species would probably be more hostile to humanity after the war due to communism.
- Any captured Covenant technology can be reverse engineered faster due it not being gatekeeped by greedy private corpos and civilians being allowed to test with de-tracked materiel (de-tracked because Rubble).
- The biggest difference I can see is the tension between Earth and Outer Colonies being less prevalent due to democratic centralization.
That’s all I can see. Idk what would be of the Halo Array or The Flood or Banished, whatever.
A socialist government may have been significantly more advanced due to factors like this.
I think the bigger point in the OP is that humanity may have de-militarized entirely. I don’t know enough about the Halo universe to get into that one.
There’s Halopedia if you want a cursory glance though you need to get actual expanded universe material like the Halo Encyclopedia in order to get a deeper understanding.
There’s no material reason to militarize because there’s barely any threats. At most the USSSDF would be a glorified peacekeeping force. That said if the USSS were to discover the Halo Array I imagine things would go similarly to how it was in the og timeline - including the Flood outbreaks.