Are there any magic items that are distinctly from the Shadowfell? Feywild stuff is easier to find but the Shadowfell is something entirely different. Is there anything from any previous D&D Editions?
In Tasha’s Cauldron of Everything you can find the Shadowfell Shard.
Tasha's Cauldron of Everything presents several shards that are connected with planes: one of them is the Shadowfell Shard:
This dull, cold crystal sits heavy and leaden, saturated by the Shadowfell’s despair. As an action, you can attach the shard to a Tiny object (such as a weapon or a piece of jewelry) or detach it. It falls off if your attunement to it ends. You can use the shard as a spellcasting focus while you hold or wear it.