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Telling a reader that the version they asked for is gone, after a hosting redirect has already moved them.

When a version is retired, the usual handling is a path-preserving 301 in the hosting config. That is the right mechanism, and it is silent: the reader clicks a link to 2.1.x, the address bar says something else, and nothing on the page accounts for the difference.

This module renders a notice that explains the move, but it does nothing until your hosting config opts in. Bumping the module pin alone changes no page.

What you have to add

Append ?fromversion= to the redirect destination, naming the version that was retired:

{
  "source":      "/agentgateway/2.1.x/:path*",
  "destination": "/agentgateway/kubernetes/latest/:path*?fromversion=2.1.x"
}

Firebase merges that with any query string the reader already had, so ?fromversion=2.1.x&foo=bar works and the reader’s own parameters survive.

The version you name must still be present in params.versions. Retiring a version means removing its content and hiding it from the picker with a blank dropdown, not deleting its entry — the entry is what makes the value recognizable. An unrecognized value renders nothing at all.

What the reader sees

Two different pages consume the parameter:

Both then strip the parameter with history.replaceState, so the URL a reader copies is the canonical one and a crawler does not see two URLs for one page. The notice is dismissible, and deliberately does not remember the dismissal: it is tied to one arrival from one stale link, not to a site-wide preference.

Translating it

Both string sets fall back to English and are overridden by an i18n key in the consuming repo, the same way bannerID works.

# i18n/ja.yaml
retired_version_notice: "リンク先のバージョン {from} は公開を終了しました。このページは {current} のものです。"
not_found_heading: "ページが見つかりません。"
not_found_lede_retired: "バージョン {from} のドキュメントは公開を終了したため、{current} に移動しました。このトピックは {current} には存在しません。"

{from}, {current}, {latest} and {productName} are literal braces, not Go template actions. Neither version is known when the page is built — from arrives in the query string at read time — so the substitution happens in the browser. Write the braces exactly as shown; a translation that drops one loses that value from the sentence.

The 404’s keys are all prefixed not_found_: title, code, heading, lede, status, try_instead, home, noscript, lede_retired, note_latest, note_ancestor_current, note_ancestor_latest, note_home_current, note_home_latest. A missing key falls back to English silently and logs no warning.

What it does not do

• No notice on a site with no params.versions. The partial emits nothing at all — no markup, no script — so a version-less site is unaffected.
• No automatic redirect from the 404. Every destination is offered as a link. A reader is already at a broken URL, and bouncing them somewhere they did not ask for hides that.
• The value is never echoed. fromversion arrives in a URL and is reader-controlled, so it is matched against your configured versions and used to select a known string. It is never written into the page.