[thrdbndndn]

Just to be clear:

OP likely copied the this share link first and then manually replaced www. to old. . But shared link never worked with old. prefix so it broke.

Reddit may or may not want to kill old.reddit but this isn't an example of it, inb4 people are going to take this opportunity to soapbox about it.

[rirrirler]

As I understand it, HN itself automatically rewrites www.reddit.com to old.reddit.com in submitted links. This works for direct links to posts and comments, but not share links (containing "/s/" in the path) like the one for this post.

[SSLy]

the share /s/ links come from the new reddit, and are a sure way to spot someone using that interface instead of the proper one.

[thrdbndndn]

Ah thanks, I had no idea HN processes links. Sorry for assuming it was the OP.