Guardians deal thorns damage to their attacker when hit by projectiles. They also seem to deal thorns damage even if their spines are retracted. This did not occur prior to 1.19.4 Pre-release 1.
Steps to Reproduce:
Hit a guardian with any projectile (or get another mob to do so). Lingering potions of harming work too.
Take damage (or watch the projectile source take damage, if it was another mob).
Confirmed; unlike the thorns enchantment, guardian thorns used to only apply to melee hits while spikes were extended.