Been lurking this thread for about a year now and finally decided to post because the Japanese Splatoon Twitter account just posted several (presumably) 4K screenshots for the third time.
The first time this happened was in July 2022 when they posted this screenshot detailing the sunken scrolls feature, which if you open the
direct link is in 4096x2304 resolution (above 3840x2160 for 4K). Due to the compression, it's hard to tell if the image was actually rendered at that resolution but zooming in on the corners of the pages shows way more detail than there would be in a 1080p screenshot.
In August 2022, they posted a
thread of images which unusually appear to have been rendered in a much higher resolution than 1080p. Splatoon 2/3 don't use antialiasing and even in official promotional screenshots you can clearly see 1080p aliasing on sharp edges, while the images posted are 1920x1080, the lack of aliasing and detail around edges suggests they were rendered at a higher resolution. Compare
this gameplay screenshot from the Twitter thread to this
1080p gameplay screenshot from around the same time, that the first image lacks any aliasing on sharp edges and the inklings in the distance have way too much detail for a 1080p image which to me look like suspiciously the image has been downscaled.
This particular screenshot is also interesting because it shows the overhead camera in the replay viewer with more detail than the current version, which forces the game to render at 720p then upscale to 1080p with an FSR 1.0 upscaler, see
this screenshot (likely to prevent the dynamic resolution scaling from constantly activating and being too distracting)
Today (May 2023) they posted these two images showing a new Salmon Run stage for the upcoming 4.0.0 patch, which similarly to the last set lack a lot of aliasing (look at the inkling on the left of the second image) and have more detail on distant objects than usual.
This isn't just a case of the developers turning up LODs or the render resolution for promotional screenshots, as everything else they put out is graphically identical to the current switch release (you can even see
some horrible low-fps Speed LODs on the jellies in the intro (something something memory bandwidth) (EDIT: was accidentally referencing a different video) and the dynamic resolution scaling being paranoid at 0:58 in
this promotional video from yesterday)
While this doesn't necessarily show the game is being developed for new hardware capable of >1080p, given how Splatoon 2/3 are capable of running at 1080p with a perfect 60fps on the current switch hardware I would not be surprised if the game was developed for both the current Tegra X1 and something new as it would easily be able to run at 4K 60fps on something more powerful, which Nintendo would love to flex.