Originally Posted by sirinsidiator
That was the case long ago, but it no longer works like that. Currently when you send a link via chat that is not one of the types "recognized" by the game, the link will not be rendered as a link and instead shows the plain markup.
The only thing addons can do is filter the chat message and re-add the link locally. That way it will show up as a custom link, but for users that do not have the addon it will look strange.
My current approach for the problem is to hijack a book link and add custom data inside. That way it will show as "[An ancient scroll]" for users that do not use my addon and as a custom link for anyone with the addon  |
I can confirm that custom links show up in the chat as |H...:...:...|h plaintext links. If you scrape the text from the chat event and print it with d() then it actually renders as a clickable link with the expected [link text] format, but the obvious downside is that the link text is still left in the public chat.
I've also never seen a custom link in zone/group/guild or any other chat from any addons. Since I can think of dozen's of uses for custom links and I'm not even creative, I think it's reasonable to assume that custom links don't work right and haven't for years...
Edit: Also thank you for taking notice. I guess I always assumed that custom links were disabled intentionally...