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TL;DR: In the past, AMD's pushed their silicon past the limits of air cooling with custom, AIO-style liquid cooling for their flashiest GPUs. Two leaked photographs indicate that they tested a similar solution for an overclocked 6900 variant, allegedly called the 6900 XTX, but eventually gave up on it. AIBs however seem to be taking over the task.
In the photos, there's a beefy, two-slot waterblock strapped down to the PCB and two thick tubes that connect it to a 120mm radiator and fan. The card's styled in the traditional Radeon colors but its industrial theme has been given a bold twist by the superhero-style "R" slapped in the center.
Apart from Radeon branding in three separate places, the card doesn't seem to have its name printed on it. It was given the XTX moniker by its photographers, who posted the two images to the Weibo and Chiphell forums, but several AMD cards have carried the XTX codename in the past so it checks out.
Last week, Sapphire announced the "Toxic AMD Radeon RX 6900 XT Extreme Edition," featuring an "XTXH" bin of the 6900 die paired with a triple-fan radiator in a closed liquid cooling loop. Because of the special bin, and excessive cooling (which even includes a vestigial heatsink and fan) the card's boost clock is 2730 MHz, a whopping 480 MHz above the reference boost clock. PowerColor and Asrock have released similarly impressive cards also based on the XTXH bin.
It's theorized that AMD chose to give their special XTX bins to AIBs to create the XTXH cards, instead of saving them for their liquid-cooled editions as they otherwise might've. Videocardz speculates that their change in plans could've been prompted by the GPU shortage.
AMD's most recent liquid-cooled GPU, the Radeon RX Vega 64 Liquid (pictured), was about 10% faster than its air-cooled equivalent, though it was a notoriously hot GPU and the reference air cooler was on the mediocre side.
It's unlikely that a liquid-cooled 6900 XT would've been noticeably faster than the air-cooled variant because the latter doesn't have any thermal problems. Nevertheless, it would've been cool to see a special edition liquid-cooled 6900 XTX, if only for the aesthetic of it.