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.