The classic Imperial void suit images for me are the original Jes Goodwin sketches of crewmen aboard Imperial Naval vessels back in White dwarf...um...I think it was either 132 or 136. That was in the original sketches for Battlefleet gothic, which took a mere 9-10 years to turn into the full game.
The suit was a bit like an old diving costume, with a heavy brass helmet with a scuba-like glass panel in it. It was all very gothic (understandably) with spikes coming off the top of the helmet. There are some John Blanche illustrations of the same style of void suit in Disciplies of the Dark Gods.
They certainly didn't look like modern space suits, and seemed to lack a clear oxygen supply. Maybe they're cheap emergency models for below-decks crewmen to protect them in the event of a depressurisation.
I imagine there are more complex models: there's another good image of a void suit used by a mechanicus explorator in the current tyranid codex, and another type of void suit illustrated in a white dwarf article from about 6-7 years ago detailing a strange FAbius Bile genetic project on an abandoned moon. Those void suit models looked a lot like the space suits usd in the original Alien movie, but with Imperial iconography. .