I don't think you could ever transport an MG in the UC; just like all open-top transports (even though squads can't fire from inside the UC). However, you can still transport MGs inside the OST halftrack.
You definitely could at some point, no one noticed or tried it because the UC cost 1,000 fuel at the start of the game, and the no garrison allowed wasp was necessary even if you bothered to go UC at all,
If it wasn't in the base game, and I'm halucinating, then it was definitely in the alpha. like for sure.