The current iteration of Tier 4 and 5 weapons lacks a slot for mementos, which makes no sense.
Especially because the current event is awarding a memento.
Cmon, Bungie.
And while you are fixing this problem, make armors able to be dyed with mementos too.
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They should just be depreciated and turned into shaders. If you've gotten it at least once as a memento, it's unlocked as a shader for you. They can have a chance to drop from activities/events for new players/players that never got them previously.