Isn't "Magus" just as much of a problem as "Janissary"? "Magus" is a reference to the Avestan priests of ancient Persia--just as much appropriation of a foreign term specifically for its exoticality, it's just been appropriated by several cultures before ours (the Greeks and Romans, primarily). But it's still appropriated and is still the word, in English, for a Zoroastrian priest, a religion that still exists. At least the Janissaries went the way of the dodo almost 200 years ago--and were never brought back.
Yet "mage" is about as deeply baked into D&D topics as any other class term--like Barbarian, which gets flak for both its etymology and its current usage. And, as noted, several of the proposed names literally mean appropriating an important religious tool for an active modern religion (Wicca, which got its start in the 1920s). Appropriation is flying all over the place here. "Janissary" isn't actually used--I mean, technically it is because it comes from the Turkish phrase that literally means "new soldier," but realistically it's not used as a title anymore--and both etymologically and, potentially, within the context of the story it could make sense to use it.* That doesn't seem to be any worse than openly stealing the tools of currently-living religions for use as weapons of war (when, as I am given to understand it, most Wiccan traditions would openly reject such violence).
*E.g., the first "Janissaries" could have been mortal conscripts taken by powerful genie-lords, but the art escaped those narrow confines. Now the term is worn with pride, as its practitioners are the "new way" of fighting, one that blends wizardry and warfare into a cohesive whole.
