So I know which URL it is now, as I've gotten a different UI in Sheets now.
[0] and [2] say the allegedly bad URL is clean. [1] thinks it isn't.
Wayback has a several month gap, before which the site is fine, and after which their provider appears to have suspended them, so presumably something happened there.
But, yeah, thanks for just shooting the wrong person, Google.
Sucks that Google isn't more transparent in its email.
[0] https://transparencyreport.google.com/safe-browsing/search?h...
[1] https://www.virustotal.com/gui/home/upload
[2] https://urlscan.io/