The resolution of MC-121733 shows that mobs taking damage from Thorns after using projectiles is intended. This does not apply to the witch, who attacks by throwing potions.
How to reproduce:
Spawn a witch
Put on a piece of armor with Thorns
Go into survival/adventure mode
Works as intended as per https://bugs.mojang.com/browse/MC-5312?focusedCommentId=191830&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-191830