Did the Bottom plate cleaning. It took me about 6 rounds until the smudges on the paper sheet became reasonably small. They didn't disappear completely though. The smudges are at the top side of the sheet, gathered to the right where the head resides.
Canon manual says:
Check the folded parts of the ejected paper. If they are smudged with ink, perform Bottom Plate Cleaning again.
That's after the first try. After the second try they recommend cleaning protrusions with a cotton swab or similar.
The manual picture generally shows cleaning whatever is accessible from the top. Given that the smudges are at the top side of the sheet, meaning wiped from the bottom of the head assembly or the head itself, I'm not sure whether the cotton swab advice applies. In any case it would require getting the head out, which I'd like to avoid.
Anyway, I did some more experiments. The paper on which I first noticed the issue and where it is very pronounced is Canon PP-201, which is Glossy II. The next examples were all printed on this paper after the bottom plate cleaning (which might not be a factor after all).
1. Printed a synthetic image with a repeated pattern of the area where on the original image the issue was the most visible. The result is a big improvement. The artifact is still faintly visible though. This was printed using media setting of PPP Glossy II A, and the matching Glossy II A ICC profile. This media was installed after the 3.010 printer firmware update some years ago. As this is a predefined media, Paper Feed Adjustment is not possible, which might be the reason for the faint vertical stripes pattern visible when magnified. See here:
https://drive.google.com/file/d/1rd4GdypMMEc6ntPU6elNHL44_6EyeYfa/view2. Printed the original image with the same media settings as above, with nearly the same result, a huge improvement but not quite complete.
https://drive.google.com/file/d/16DF9FYXT6KjcVyLIJMhpqPOjZzxlvj9o/view3. Printed the original image using a custom media which resembles the settings of PPP Glossy II A as much as possible, with the Paper Feed Adjustment done, so should be the best configuration. Yet the artifact is back again at full scale.
https://drive.google.com/file/d/1m_nKYjZofUcAnJcX9s2dZnpLuzu6_1Dw/view4. Printed the original image with the same custom media from Photoshop's Print menu (the others all printed from Professional Print and Layout PS plugin). The improvement is there again to the same extent.
https://drive.google.com/file/d/1hmmcjSlG12PsdodcezL_xcrfxlWDf4uD/view---
At this point it seems reasonable to assume the issue is not on the hardware side. The nozzle check is good, the maintenance and cleaning rounds done, and the printer proved it
can print the area almost fine at the same spot, and completely fine elsewhere on the page.
There is a chance it might be a transport issue due to the persistent location of the artifact, but then it shouldn't vary as much with the media setting, should it? Especially with the print head setting, hmm...