Hi @SidHiscox,
Thank you for your question. Oh no - that sounds like a frustrating experience trying to find it!
Could this perhaps be live in a flow email somewhere like your abandon cart? I would see if you can isolate it to an example account of a customer and figure out what emails they have received.
Also if that page is no longer relevant I would suggest creating a 301 redirect at least on the page so when someone clicks it they are directed to a more relevant page.
Lastly - if you run your emails through a tool like Litmus this will pick up all the links used in an email (as well as many other handy pieces on your email quality). This may also be worth doing to try and find it!
Hope that helps you on your way.
Thanks
David