It is strictly in the FRA rules but I think that EOs bend them on this one, especially in the age of spinners. A chunk hit off in spinner blow is in my opinion likely to do more damage than an axe head falling off or being thrown.
Battleaxe had a chain tether at Insomnia last year and this was accepted in tech check though it broke in the 722 battle when the head came off anyway :/ It's now on a steel cable tether. Broadax on the other hand is one-piece Hardox so I don't believe it will be required depending on the EOs discretion. Though if in any doubt about the robustness and safety I would add a tether. You are right as far as I know the last Hatchet revision wth the one-piece also did not carry a tether.
It might be the case of still being needed for arenas without a roof I guess.
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