Street Fighter style hitboxes.

What would be the best way to go about creating these types of hitboxes for a melee combat system? As you see in the youtube video above the hitboxes move and change size during the animation. So it wouldn’t something simple like attaching a cube collider to socket. The only thing I can currently think of would be have many different hitboxes and to enable/disable them based on current animation.

You can animate position, orientation, and scale.
All you need to do is create bones in your animations that are specific to the hit boxes, and make them animate as appropriate (assuming some standard size hitbox.)
Then attach collider boxes of the appropriate default size to those bones when you build the character, and it should Just Work (tm.)
You could name the bones something special (like “HIT_…”) so they’re easy to find, and you could have a loop over bones that makes and adds collider boxes to any bone called HIT_whatever, if you don’t want to manually tech each one.