Ah - I actually moved away from Memos (I just wasn't using it and it was taking resources for no benefit) so I can't actually directly help, buuuuuut I want to be optimistic and assume that the 'feature' was removed given the page 404ing.
You could use something like NextDNS (or any other DNS solutions that offer logging, I just use ND on my network), set up as the server's DNS provider, and see if the domain (above) shows up in the logs after a couple days. Though if you're running other software alongside it, then it might not be from Memos, which can lead you down a 'disable, wait, check, repeat' rabbit hole. But that's how I would do it, myself.
Sorry I don't have a quick and easy solution for you :(