Spoilers are ahead. Towards the end of the Netflix mini-series Halston, a character chides Halston, as played by Ewan McGregor, about his voice. Roy Halston Frowick grew up in Iowa, so why does he talk the way that he talks? That moment almost breaks the fourth wall, because the folks at home have surely been wondering what's up with McGregor's accent. McGregor's imitation of the real Halston's posh voice is a bit exaggerated, possibly for dramatic effect, but it's not that far off from how Halston really sounded.
There has been some criticism of McGregor's casting in this role, because he is a heterosexual man playing a gay man whose sexuality is a massive part of his story — a trend among Ryan Murphy-produced projects. The voice, however, is not an blunt affectation. McGregor's accent in this series is an imitation of the very same accent that Halston put on in his professional life.
Halston did not speak with a midwestern accent in real life, despite growing up in the region. Instead, the real Halston adopted what is known as the Transatlantic American accent — or sometimes Mid-Atlantic Accent — an affected way of speaking that was favoured by early Hollywood stars in the '30s and '40s, and had mostly died out by the 1950s, though some like Halston hung onto it. It's an accent that was taught, rather than one that occurred naturally — in fact, another Murphy series on Netflix called Hollywood actually has a great scene in which the young actresses at the studio receive formal Mid-Atlantic accent training from an instructor.
And you've definitely heard a version of this accent before. Katherine Hepburn and Cary Grant are practically synonymous with Transatlantic Accents, so if you've ever gone through a TCM phase, you've heard it.
Hayley Mills' New Englander character in the original The Parent Trap is another great example of the refined-sounding accent; in fact, there's a scene in that movie wherein she has to teach her more relaxed Californian twin to speak like her. The most modern example of the Transatlantic accent in something that's not a period piece is probably Frasier.
Halston belongs in this of examples as well. In a 1987 interview with Halston and Liza Minnelli (below), you can hear his way of speaking. (You can hear a sample of the real Halston's voice in a commercial for rugs that, incidentally, is dramatised on Halston.) Note how he pronounces words like "couture" and "clientele."
So, for what it's worth, Halston essentially got the accent correct. McGregor's change in voice is more notable than, say, Krysta Rodriguez's more subtle imitation of Minnelli's accent and voice, but it's not incorrect. There are plenty of other liberties taken with the show's depiction of real events and Halston's life, but the voice is the real (affected) deal.