Looks like a OnePlus 5T is indeed happening.
OnePlus CEO Pete Lau today took to his company's official forums to explain why the OnePlus 5T has a headphone jack. OnePlus hasn't yet made any official announcements about a "OnePlus 5T", so it looks like this serves as that.
As for why OnePlus will continue to stick with the 3.5mm jack while other companies are ditching it on their flagships, Lau gave a couple of reasons. First is that 70 percent of the OnePlus community values audio quality, and a 3.5mm jack lets them use any headphones on the market, including high-end audiophile ones.
The second reason is user freedom. Lau explains that while ditching the 3.5mm jack and using USB-C for audio would let OnePlus slim down its phones, this change would not noticeably improve the user experience, so it's not worth the trade-off.
Lau didn't reveal any other details of the OnePlus 5T, saying only that OnePlus is "confident that our product decisions for our upcoming device will offer the best flagship experience possible."
An image that leaked earlier this week suggests that the OnePlus 5T will have super slim bezels around its display, like many other flagships that've been released lately. Few other details about the 5T are known, but now that OnePlus has officially confirmed the phone's existence, a full reveal may not be far off.