with the exact same restrictions and semantics as the Location HTTP header
Maybe that should be 'syntax'? Semantics would mean that the header behaves the same way right? But it doesn't. Location is a prompt-less redirect, O-L is a prompted redirect. Additionally, O-L has an additional restriction that the URI specified must be .onion?
websites with lots of client traffic are encouraged
Why do we need to encourage them? Aren't they sufficiently motivated themselves? I would go so far as to suggest they do _not_ do that, because there is no fully reliable detection mechanism. But if they want to, we 'can provide suggestions for them'?
And perhaps one suggestion is to detect User-Agent and only serve it to one of the five user-agents that support Tor? (Since we discourage anything else?) (TB, TBA, Brave, Orfox, OnionBrowser)
-tom