I’ve seen countless posts pointing out that it’s odd Master Chief’s Mark VI Armor isn’t in the MP and others saying it is.
No it isn’t.
Master Chief’s armor used in campaign has never been available in any of 343’s Maps. Chief’s Mk VI Mod Armor set is very noticeably different from Halo 4 's Gen 2 Mark VI set. Chief’s Gen 2 Mark VI Mod is different than the ones in H5. Same with his cracked visor. And his Gen 3 Mark VI do not even have an equivalent in Infinite. The Mark VI is available for the Mk V[B] core. But that is very different.
Would it be nice to have Chief’s Armor, yes.