(This is somewhat late, but I haven't seen any mention of it.)
A little while ago (before February, I think), TeXdoc.net changed their URL structure. Before, the link to the TeX WEB
documentation looked like this:
http://texdoc.net/texmf-dist/doc/generic/knuth/tex/tex.pdf
So it basically mirrors the TeX directory structure.
Now that URL redirects to https://texdoc.org/missing.html, which I suppose is their 404 page.
The correct link is
https://texdoc.org/serve/tex/0
I haven't done extensive testing, but it appears that the segment after serve
is more or less what you would look up, and the number selects one of the results you get. For example, searching for tex
yields tex.pdf
as the first result, and the second is the documentation for a package called mensa-tex
; the link for mensa-tex
is indeed https://texdoc.org/serve/tex/1.
In any case, all existing TeXdoc links from before the change, or at least all of the ones that I have encountered, are now broken. Tell your friends? I know that it is commonly assumed that everyone here has a TeX distribution ready; however, TeXdoc is quite handy if you are on a mobile device or if it is simply not feasible for you to have TeXdoc and all the documentation installed (e.g., if you don't have a lot of disk space). Some of the documents are available in PDF form from CTAN, as an alternative, but others are not.
I admit that this isn't a question. It's also not a real issue: There is neither a problem with this site nor anything wrong with TeXdoc's new design, per se. (Although I can't help but feel that both URL structures could be supported.) Since TeXdoc is probably rarely used by most contributers here (I could be wrong), the change might not have been noticed. If/when there has been a mass link breaking in the past, either on TeX.SE or elsewhere in the network, what has happened?