EDIT: this post is incorrect, this workaround did work but I'm not sure why. still looking into it, having custom_skin as a member variable seems to help

I did manage to find a workaround, if you do all the find_skin methods as onready vars, then you can compose the new_skin with any of those variables with no crashing. The new_skin can have as many of the onready vars added to it as are required.

extends SpineSprite

var r_true=true
var custom_skin
var new_skin
onready var r_skin= get_skeleton().get_data().find_skin("r")
onready var g_skin= get_skeleton().get_data().find_skin("g")

func _on_skinswitch_pressed():
   if !r_true:
      custom_skin = new_skin("new")
      custom_skin.add_skin(r_skin)
      get_skeleton().set_skin(custom_skin)
      get_skeleton().set_slots_to_setup_pose()
      r_true=true
   else:
      custom_skin = new_skin("new")
      custom_skin.add_skin(g_skin)
      get_skeleton().set_skin(custom_skin)
      get_skeleton().set_slots_to_setup_pose()
      r_true=false

The problem comes into play when you try to find_skin and add_skin in the same frame, from what I can tell.

Luckily this workaround is pretty easy, although I guess it requires more memory since you have to load all the skins you want to use for a SpineSprite on creation rather than just when you'd need it. You could also set up the game loop to find_skin, wait a frame, then do new_skin if that's viable.