I am not sure if automatic deletion of some "low-quality" posts (such as heavily downvoted, without reply, closed, ...) is a configurable feature on individual stack exchange sites.
Voting is very subjective. Many "emotional" users cast their votes (up or down or close or delete) irrationally. Voting is also like an inflectional disease, and many like to follow some lead or majority without adequate judgement. Some "low-quality" posts (many of mine) are in fact victims of malicious or ignorant downvotes and close votes with stereotyping. Some "high-quality" posts (many of mine) are in fact not worth that many upvotes, but may be because of some good replies, or being highly exposed on twitter, or having stayed for a long period, whereas I just need a proper answer.
Regardless of whether a post is truly low-quality or not, every post (except extreme cases, such as Unix & Linux unrelated, or offensive posts) deserves their existences, and visibility to the public. Their existence have values. Duplicates can help us to recognize the different appearances of the same or related problems. Victim posts will still have chances to be re-recognized. The posts of a user can be kept tracked of by themselves or others for reducing the possibility of making the same mistakes or asking the same questions.
In the meantime, I am not sure if SE sites might have some technological difficulty of keeping every post's existence and visibility, but I just can't imagine there is one (limitation on storage, querying processing, ...).