This API is removed now, use the site settings
https://github.com/discourse/discourse/commit/e283e6aea002fb533640e68f8ee7447cf0a7c90d
I am open to allow allowing a proper plugin api, but it can not be done as a global the way the old one was.
As it stands plugins can already hoist out html if they want to so its not particularly interesting to also allow them to whitelist iframes.