With the usual caveat that I'm not a lawyer, I think it works something like this: when someone submits content to SE, they are doing so under the terms of the CC license, which means that if SE chooses to redistribute that content, they must do so under the same license. However, SE is not under any obligation to distribute the content. When a post is deleted, SE is simply declining to redistribute the post's contents to anyone other than moderators and 10k users. But as far as I know, the terms under which SE distributes deleted content to moderators and 10k users are no different from the terms under which SE distributes undeleted content to everybody. (And in fact, I'm pretty sure they can't be, since SE received the content under the SE license in the first place and is thus not authorized to distribute it under any other license.) So the attribution requirements for a deleted post would be the same as if the post weren't deleted: you have to identify the source and the author's account name, and link to the source and author's profile if it's in a hyperlinkable medium. For deleted content, those links may be broken, but they're still legally required. Even if you think the OP may not want to be attributed, you're still required to do so - the alternative is that you don't redistribute it at all.

If the account has been deleted such that you actually can't get a URL for the user's profile, I think that technically means you can't redistribute the content at all. After all, the general rule with licenses is that any redistribution (like any of the other behaviors protected under copyright law) is illegal by default, but you get an exception that allows you to do it as long as you are able to fully comply with the terms of the license. "Best effort" or "do as much as possible" compliance doesn't count.

Of course, note that just referencing an answer is not a reserved right under copyright law (in the US at least). You'd have to actually copy some amount of it, or share something that qualifies as a derivative work, which means that it's substantially based on the answer, more or less.

But like I said, I'm no lawyer, so perhaps someone from SE or someone who's more familiar with the licenses will clarify/correct this.

With the usual caveat that I'm not a lawyer, I think it works something like this: when someone submits content to SE, they are doing so under the terms of the CC license, which means that if SE chooses to redistribute that content, they must do so under the same license. However, SE is not under any obligation to distribute the content. When a post is deleted, SE is simply declining to redistribute the post's contents to anyone other than moderators and 10k users. But as far as I know, the terms under which SE distributes deleted content to moderators and 10k users are no different from the terms under which SE distributes undeleted content to everybody. (And in fact, I'm pretty sure they can't be, since SE received the content under the SE license in the first place and is thus not authorized to distribute it under any other license.) So the attribution requirements for a deleted post would be the same as if the post weren't deleted: you have to identify the source and the author's account name, and link to the source and author's profile if it's in a hyperlinkable medium. Even if the deleted content is not accessible through those links, it would seem they're still legally required.

However, if you can establish that the OP does not want the content to be attributed to them, that would probably qualify as a request to remove attribution under section 4(a) of the CC license, and under that circumstance you could redistribute the content without attributing the poster.

In the CC-BY-SA 3.0 license, you're required to provide four pieces of information for attribution:

1. the name or pseudonym of the author or entity designated by the author as the attributee, if available
2. the title, if available
3. the URI specified by the licensor (SE) to be associated with the content, unless it doesn't refer to the license information
4. if you adapt or incorporate the content into something else, a notice of how the original content was adapted or incorporated

If the account has been deleted such that you can't get a URL for the user's profile, since item (1) says you only have to provide the name if available, you can probably just go ahead and do it without attribution to the original poster.

Of course, note that just referencing an answer is not a reserved right under copyright law (in the US at least). You'd have to actually copy some amount of it, or share something that qualifies as a derivative work, which means that it's substantially based on the answer, more or less.

But like I said, I'm no lawyer, so perhaps someone from SE or someone who's more familiar with the licenses will clarify/correct this.