It’s fairly common to automatically close discussions after they have gone a few weeks (or months) without a response.
The benefit of this is it keeps the community fresh.
It’s tiring to see newcomers responding to discussions that haven’t received an update in several years. The common case is the member arrives via search to an outdated post, tries the advice provided, and points out it no longer works.
If the discussion was closed for new posts, that’s a good indicator that it’s out of date (or the matter is ‘settled’). It would also encourage the newcomer to create a new post that might solicit more recent, up-to-date, information.
The downside of this approach is you’re likely to get a lot more repeat discussions on the same topic. It can be exhausting for members to repeatedly answer the same questions.
So, what’s the best option? In most cases, it’s usually best to close discussions that haven’t received a new update in several weeks. For extremely large communities where repeat questions would be a problem, it’s probably best to try and have ongoing, definitive, threads (and up-to-date knowledge-base articles), which members can reply to.