That index page hasn't been been altered since 2016.

The site it links to under PKS and PKB, danceandmusicindexes.org, has expired:
http://www.danceandmusicindexes.org/DFIE/Source0/S0001865.htm

There's an archive of that site here:
https://web.archive.org/web/20141217022316/http://danceandmusicindexes.org/

But it doesn't include that page. Here's a list of the pages it does include (we want #1865):
https://web.archive.org/web/*/http://danceandmusicindexes.org/DFIE/Source0/*

Taking #1679 as an example, here's what that database held for that dance:
https://web.archive.org/web/20150529022214/http://www.danceandmusicindexes.org:80/DFIE/Source0/S0001679.htm

So it's a link to a book rather than instructions for the dance. My guess us that the reference to the book with the Mildmay dance was not archived.