This points already to the internal link: https://en.wikipedia.org/wiki/MediaWiki:Histlegend Maybe https://en.wikipedia.org/w/index.php?title=English_Wikipedia&action=info#mw-pageinfo-watchers is enough?
There are user scripts that still reference this old tool: https://en.wikipedia.org/w/index.php?search=mzmcbride+watcher+intitle%3Ajs&title=Special:Search&profile=all&fulltext=1. As Jaime notes, we have https://en.wikipedia.org/w/index.php?title=English_Wikipedia&action=info#mw-pageinfo-watchers now. My guess is that some custom user script that @Samtar loads was recently updated to remove the (broken) link.
Quite possibly! Are there any plans to fix the tool? If not, redirecting from the tool to index.php?title=[title]&action=info#mw-pageinfo-watchers could be a good work around? (Would obviously be preferable to remove the links)