On source URLs

I thought it was stated somewhere on the site that image source URLs should be to a specific page on which the image is seen: not a direct link to the file, neither a link to an unspecific page such as a gallery index or an account’s main page. However, I can’t currently find anything like that, not in the rules, FAQ, “about uploading” page, or their revision histories…am I missing it, or am I just imagining ever seeing it? Maybe I’m remembering something from Ponibooru?

 
I personally can’t remember, if this was stated anywhere. Maybe it was on a “expected as a good practice” level, since there is a error-level tag, which indirectly applies to the whole query you linked.
 
I believe, tagging sub-forum will fit the best.

Do mods/admins have the power to change sources programmatically? I don’t see a way to set source URLs via the derpi API.
 
Certain URLs need updating because the host platform has changed access patterns, for example, with deviantArt, URLs with a /#/ before the path just need the #/ deleted, and URLs using the shortened 7-character identifier can be changed to http://fav.me/???????, but it’s a bit daunting to do so for ~1500 such images. (Of course, this doesn’t guarantee the original page hasn’t been deleted.)