Cannot put Soulbound enchant to Iron's spell spellbooks, all other curios items can have soulbound enchant.

Neoforge 21.1.72
ars_nouveau-1.21.1-5.2.2-all
ars_elemental-1.21.1-0.7.0.8

In 1.21 the criteria for enchantments compatibility is tag based, i could sneak in the curio tag they use i guess