Route each out-of-stock page by whether the product is coming back, and you avoid both costly reflexes. The blanket habits, “always 404 out-of-stock pages” and “always keep them,” are wrong in opposite directions: one throws away pages that still hold traffic and links, the other clutters your catalog with dead ends. The right disposition depends on the product’s return status and the value the page still carries. There are three branches, and each has its own test.
Keep the page when the product is temporarily out or genuinely returning. If it sells out seasonally, ships again soon, or simply lapsed between restocks, the page should stay live and indexed, with the availability marked honestly and, ideally, a way to be notified or to see related items. This is also the right call when the page holds real value: meaningful organic traffic, ranking positions you would lose, or backlinks pointing at it. Killing a page like that discards equity you would have to rebuild from scratch.
Redirect the page when the product is permanently gone but a relevant replacement exists. Send a 301 to the closest equivalent product, the newer model, or the parent category, so the traffic and any link value flow to a destination that can still satisfy the visitor. The test here is whether something genuinely relevant exists to receive the redirect. A redirect to an unrelated page is just a disguised dead end and frustrates both the visitor and the search engine, so only redirect when the target is a true alternative.
Noindex or serve a 410 when the product is dead and nothing equivalent remains. If it is discontinued, has no successor, and no relevant category to absorb it, there is no reason to keep it in the index. A 410 tells search engines the page is intentionally gone, and a noindex quietly drops a low-value page that still needs to exist for some internal reason. This is the branch for pages with no return, no replacement, and no remaining value.
Work through your out-of-stock pages one at a time and ask two questions of each: is the product coming back, and does the page still carry traffic, rankings, or links? Keep the ones returning or valuable, redirect the permanently gone with a real alternative, and noindex or 410 the truly dead. Decide per page, not by policy.