It does not look like the prefix is inserted automatically. For example, recently I saw the post "How should i document my code?" (
https://news.ycombinator.com/item?id=13517171) appear in the main feed. That post also appeared in the "ask" feed, even though it did not have the "Ask HN:" prefix. This tells me that the prefix was not inserted automatically, and since the post I referenced wasn't flagged/killed or otherwise hidden, I think text posts without the prefix are OK.
That being said, I think at this point the "Ask HN:" prefix has become a site convention, and so it's worth putting in the prefix. It also helps make clear which posts are links and which posts are questions or discussion topics.
P.S. Yes, I'm aware that non-links don't have a domain name after the title. But I argue the "Ask HN:" prefix is better: The domain appears after the title, and since titles all have variable lengths, your eyes have to search in two dimensions for the domain; with a prefix, your eyes just have to move in one direction.