Is 老鸽 (lit. "old pigeon") slang? I've seen some cases (in stories, on the internet) where people call each other '老鸽'. As the literal meaning 'old pigeon' doesn't make much sense, I think it's slang but I haven't been able to find out the meaning.

So what exactly does '老鸽' mean?
老鸽 is an intentional typo.  鸽(Pigeon) can mean "get stood up"(被鸽了，被放鸽子) or "stood sb. up"（放某人鸽子）.  So this is the combination of both 'bro' and 'a guy that often does not show up in appointments'.