A discussion in comments got deleted. Why? Can I see a copy for posterity somewhere?
(If you've been linked to this question, it's because you asked about some specific instance of this - it's a common one.)
Comments are meant to be at least potentially temporary, and when they're deleted, they're not coming back:
Comments are temporary "Post-It" notes left on a question or answer. They can be up-voted (but not down-voted) and flagged, but do not generate reputation. There's no revision history, and when they are deleted they're gone for good.
There are several reasons comments may be deleted; you'll see them when you go to flag a comment. It could be rude or offensive, not constructive, obsolete, or too chatty.
Some combination of not constructive and obsolete is generally the reason that large swaths of comments (some sort of discussion) get deleted. The general motivation is that we don't want future readers to have to wade through some kind of irrelevant discussion, only to realize that they haven't actually learned anything - it was just some side debate.
Yes, on cooking we're a bit more proactive about this. We have users who are vigilant about flagging obsolete/not constructive comments, and we mods do our job and handle them appropriately. Remember, StackExchange sites are Q&A sites, not blog/discussion/chat sites. In the end, we want all the content on the page to serve future readers of that question and its answers - so comments which don't help serve that purpose may be deleted. For example, if someone asks for clarification, and the post gets edited to clarify, that comment is no longer necessary. Similarly, if there's a debate over whether frobbles are fribulous or frobulent, and it ultimately gets resolved and the post is edited, the debate doesn't need to stay there.
We will never delete comments which add something to a post. If you feel that's happened, by all means post here on meta (or find us in chat) and we'll sort it out.
See also this question for where you should have your discussions in the first place, especially if you want them to have some permanence.