By "error images" below, I mean the images used on each site for the 404 Not Found page, the 500 server error page, and the captcha page. I'll admit that I'm a bit biased as I strongly believe that each site should have its own, unique set of error images, as they add a certain level of charm to the site's designs, which I don't want disappearing.
Back when the sites were being redesigned last year, the original plan was to unify these error images across the network. In response, I had requested that the team consider allowing sites to continue having their own, unique error images, and that request seems to have been granted: all of the existing designed sites got to keep their site-specific error images.
However, I later found that the Code Golf site, a site that received its design after the new design layouts were adopted, didn't receive custom error images. When it received its design, there were no custom images where they were supposed to be, resulting in those image URLs serving 404s themselves. Later on, staff changed the pages to force-load the images for beta and non-designed sites.
After some further searching, I found that none of the sites that went from being non-designed to designed after the responsive design layout changes received customized error images, which have historically been deployed when a site proceeds to the "designed" phase. Anime & Manga, one such site, was told the following regarding this:
404, error, and captcha custom designs are out of scope for new designs, I'm afraid.
Why don't newly-designed sites get custom error pages anymore? Is it because of a lack of designers?
And if the plan is to not design custom error pages for sites anymore, will community submissions of custom error images still be accepted?
(Side note: while Stack Overflow did lose its "polyglot" 404 image, that is irrelevant to this question, as it still otherwise has a site-specific set of error images. That was just the custom images changing on one site, and not a reflection of what happened network-wide.)