Apparently, the host 'tools.aftertheflood.com' is still mapped to GitHub Pages.
Backing out from the CSS file (many thanks!), turns out the top-level web page is still up -- BUT the links to download the zip file font collections DONT WORK.
So you still need to scrape the CSS file to get working links to the individual fonts... (I was going to post those URLs here, but there are 84 of them. Use a command like this to scrape the CSS file:
grep -o "http[^']*" < sparks.css
However, this GitHub Pages site still hosts web pages that show some great examples and links to ObservableHQ notebooks (which also still work, hosted at observablehq.com).
Very neat idea, though! I did find this example: https://observablehq.com/@tomgp/sparks-with-live-data
For those examples to work (which they still do), the URL references to the font data in the stylesheet should ostensibly still be valid: https://tools.aftertheflood.com/sparks/styles/font-faces.css