The current 1.8 spec should feature a more prominent warning about being outdated

[slikts]

This relates to issues like #15711 and #21477, and the suggestion is to add a more prominent notice that the current spec is long out of date or legacy. A more visible notice is needed because the document's presence in the master branch under doc/spec.md suggests (at a glance) that the document would be current, and the version number and date at the top is easy to overlook and is bound to cause confusion (speaking from experience).

[jakebailey]

This is an old issue, but in #51791 I deleted the spec wholesale; I don't think it gets more prominent than "doesn't exist".