If a community around a topic of a site is no longer actively maintaining or building the body of knowledge each SE site wants to be, then a site will be closed and archived.
From Graduation, site closure, and a clearer outlook on the health of SE sites
If a public beta site does not produce consistently helpful content, and lacks the caretakers needed for flags and spam to get handled and our Be Nice policy to be upheld, it will be closed.
Before that decision is made a community manager will probably reach out on site meta and see if there is anything they can do to revive the sites topic. Tweaking the site scope can be considered. Or a rename.
If the options have run out then the site will close. This happened before (twice on the same topic) and will happen in the future. The questions and answers are preserved in a datadump.