1) the IE default page gives additional information what could cause the problem and how to maybe fix it. And it is localized to the users language. As such IE's page is much better for a non-technical user and doesn't at all impede the technical user.
2) the moment that page gets customized with all that info you propose it will be longer than the 512 bytes at which point IE will not display its customized error page.
Plesk's solution here does nothing but make life harder for non-techies (which admittedly don't use plesk) for no reason what so ever. The page served in the example is inferior to IE's page in all aspects. Hence my snarky comment
1) Not all defaults are awesome. The issue here is that this is not even a site default, this is a default for a multi-website management program.
And for that matter, if you leave the wrong settings unset on recent versions of IIS, this message won't go out anyway, since IIS will push its own default page for non-200 responses.
2) It's trivial to have a simple, CSS-based site layout that requires less than 512 bytes in the base HTML page. For a simple notice that this isn't a known page on the site, but here's a search form, you could easily still be under 512 bytes.
2) the moment that page gets customized with all that info you propose it will be longer than the 512 bytes at which point IE will not display its customized error page.
Plesk's solution here does nothing but make life harder for non-techies (which admittedly don't use plesk) for no reason what so ever. The page served in the example is inferior to IE's page in all aspects. Hence my snarky comment