Ok, thanks, I’ll see if I can contact Microsoft; I’ve not had a good history with their support though.
The link I sent, I also got from the waybackmachine
Oh, did you? Your link is of the original page and not a waybackmachine link. Likely what you did was you copied the link from the in-website address bar by mistake. The address contained in it is the original version of the address; not the archived one. So to copy the actual waybackmachine archived version of the link, you need to copy the address from the web-browser its selves own address bar. I made the same mistake in my last comment, and I had to edit it to fix it, because the links were directing to the original pages rather than the archived ones which I had manually saved.
I found an official guide when hunting around in the Minecraft Help Center, but it also seems to not work just like the unofficial one I mentioned. It says to go to the 15th anniversary page (the one that currently tells us about buying tickets for Minecraft Experience) and scroll down to the “Get the 15th Anniversary Cape!” section, which doesn’t seem to exist anymore. So I think this is just a case of Mojang not updating their help page’s information. I decided that as much as I really want the cape, I can’t be bothered going through Microsoft Support for it; Microsoft Support just sucks, lol.