I sure hope they won’t take down old.reddit.com. IMHO, Reddit would be unusable without it.
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I have no idea if it was an advertised or officially supported scheme. And I don’t know what the nature of the URL redirect/rewrite/forwarding was. It was handy for remembering. Now all those links seem to redirect to the main domain.
Old.reddit still works but could go soon if they’re reorganizing schemes/standards.
I actually used that subdomain structure years ago to allow users to select different subset styles for subreddits.
Old reddit is still fine for reading content though.