As far as I can tell, there is no evidence that this is in any way an "official alternate costume" and especially not from FEH.
According to DeepL, the commentary reads "Fuka's Assassin costume definitely looks good on Kamui! *Caution against costume alteration."

The two kanji "風花" are in fact the first two kanji of FE3H's Japanese name, Fuukasetsugetsu, and this combined with the obvious generic class outfit indicate that this is simply a cosplay of the Assassin class from Three Houses and not in any way official; even Byleth wearing the same clothes would not count as an official alternate costume, as she was given an Assassin-based outfit in a Cipher card (post #3716832, post #3850796) which does not resemble this. No official media except for the concept reference sheet of the class (https://imgur.com/uEQ3tPC) actually depicts any form of the generic Assassin class outfit. As such, I have removed the tags official_alternate_costume and fire_emblem_heroes from this post. The best tag to use would be "assassin (fire emblem: three houses)" in the same vein as the tag "dark mage (fire emblem fates)" but no such tag exists, perhaps for good reason.