Looks like the Nightly pref-flip that enabled this was in bug 1719271, which landed on 6/23. I'm not sure if there's a bug for preffing this on for release, so I'll just mark it as depending on the metabug `https-first-mode` as the underlying platform bug for now.
Bug 1868167 Comment 5 Edit History
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Looks like the Nightly pref-flip that made this start working was in bug 1719271, which landed on 6/23 (though it's using an `IS_NIGHTLY_BUILD` check for now; this isn't riding the trains yet). I'm not sure if there's a bug for preffing this on for release, so I'll just mark it as depending on the metabug with alias `https-first-mode` as the underlying platform bug for now.