Previously we had Firefox implementing and Chrome with an in-progress implementation, which was enough to keep it in the spec. But Chrome has just WONTFIXed their bug for context menus, and removed the related code: https://bugs.chromium.org/p/chromium/issues/detail?id=87553&desc=2#c66

Unless there is strong interest from Safari (@rniwa? @cdumez?) or Edge (@travisleithead?) for implementing these, we should remove them from the spec, as they do not meet the bar for features to be included in HTML.

Sincere apologies to Firefox for taking a risk that didn't work out. As detailed in https://whatwg.org/working-mode, this kind of feature without multi-implementer support should not make its way into the spec in the future.