#12251 removed explicit definitions of ::file-selector-button as an element-backed pseudo-element in CSS Forms 1 and CSS Pseudo 4. However, it is still defined in § 5. Element-backed Pseudo-Elements in CSS Pseudo 4.

I think fixing this editorial issue would also clarify whether ::file-selector-button is also no longer a tree-abiding pseudo-element, since all element-backed pseudo-elements are tree-abiding pseudo-elements.