You ask specifically for policy, but I'm going to approach this from a more systemic standpoint, particularly concerning downvotes– based on how the site works, no, it's not broadly discouraged, and would fall more into a site norms area. If anything, I'd argue that current site design actively encourages this behavior through its incentive structures.
As it stands:
Particularly with the latter, just from an incentives standpoint it seems crystal clear why people would want to do this: they tangibly benefit! The more critical the voting feedback, the more reason to wipe the slate clean.
But zooming out a bit, if we view the goal of Stack Exchange sites as "building a library" (or many), then if I post something in earnest that other users signal as a bad contribution, then self-removal arguably isn't just sensical for me, but actively beneficial to the library! I don't think it's unreasonable to assert that we don't benefit from "bad" answers, in the general sense.
Given, I'd also say that fixing is superior to self-censorship. But depending on the site and the context, that may range from easy to downright impossible, especially on sites scoped to a less objective subject matter than, say, programming, or if my answer turns out to be just incorrect.
In practice, there's nuance here, and things are downvoted and deleted for a myriad of reasons– I'm not privy to the specific case referenced in the question that produced the quoted comment, and I can definitely imagine a hypothetical where a user deletes their own post in the midst of otherwise acting poorly.
But overall, I just find it really difficult to see this as a bad behavior when I look at how the site currently functions and rewards users; much less call it "toxic" for a user to remove something that the community around them has explicitly signaled as poor, low quality, and/ or low utility.
With no apparent incentives or guidance to the contrary, it feels like a humongous ask to expect anyone, old or new, high or low rep, to want to weather downvotes when they're given little to no reason to do so.
It's a valid caveat, however, that "negatively received" may not necessarily equate to "negatively scored". If, for example, a user is unable to take genuine, constructive criticism, or they rollback helpful edits, or delete their post after meaningful refinement by the community– that feels more legitimately classifiable as "toxic" to me. Collaboration is the name of the game here; it's woven into the DNA of SE sites, and it's not something a user can opt-out of.
Some other thoughts that came to mind that didn't weave well into the above:
- The asymmetric reputation impact of votes (+10/-2), I think, does reward salvaging answers over deleting them, since it only takes a single upvote to nullify the reputation impact of five downvotes
- I'm definitely not saying that the current design can't or doesn't necessarily have negative side/ knock-on effects
- Whether site design should be this way, or whether it could be better if tweaked to some degree, is a worthwhile, but separate, discussion
This post is an expansion of a comment I made under @JourneymanGeek's answer.