When using " /item modify entity @s weapon.mainhand {"function":"minecraft:set_components","components":{"!minecraft:equippable":{}}} " while holding an armor piece that has the equippable component it will not remove it.
Instead it will get replaced with a !minecraft:equippable": {} component and the armor piece cannot be equiped anymore.
When doing this with any other item the component does get removed instead of replaced.
It is possible this was also precent in previous version, 1.21.4 is the only version I have tested it in.
The
!minecraft:equippablecomponent that appears is what confirms you have correctly removed theminecraft:equippablecomponent from the item. Since armor items include this component by default, the absence of this component from the serialized data would cause the game to interpret the item as though it does have the component, with its default values, so if the component is actually missing, this should be saved on the item's data in some way.