With most religious dress the restriction is on wearing. With headdresses the restriction often extends to showing, and that has consequences for how a reference work can handle them.
The Plains war bonnet is the clearest current case. It is earned rather than acquired, each feather traditionally corresponding to a specific act, and it belongs to particular nations rather than to Native America as a category. Its wearing by people outside that context, at festivals and in fashion, has been objected to consistently and publicly by the communities concerned, and several institutions and events now prohibit it. Reproducing it as decoration participates in the same appropriation.
In Vajrayana Buddhism, crowns with five panels bearing the five Buddha families are used in empowerment ceremonies, and the details of what is done with them belong to the transmission and are not published. A description of the object is available; the practice is not, and the tradition's own position is that publishing it would not help anyone.
Egyptian crowns present a different kind of absence. The white crown of Upper Egypt, the red crown of Lower Egypt and the combined double crown are known from thousands of images across three millennia, and not a single physical example has ever been found. Whether they were made of perishable material, buried in a place not yet located, or ritually destroyed, is unresolved.
The atlas therefore carries no drawn glyph for this entry. Some of what would be drawn is restricted by the traditions that own it, some is known only from depictions whose accuracy cannot be checked, and a composite invented from several traditions would misrepresent all of them. That decision is recorded here rather than left as a silence.