This is on the WoW anniversary vendor Historian Ma'di. It's a cosmetic cloak appearance that can be learned, but ATT isn't tracking it as learnable. I'm unsure if it's a new item for the 16th anniversary or if it's been around in the past.

Update: Blizzard said the cloak was new.

It is new item this year and it was added to ATT today. It will show up as (un)collected on the item tooltip, however it refuses to show up in my lists unless in Account Mode right now, still investigating that.

You can download latest in-development version and put it into your addons folder manually here.

Everything should be fixed as of acd9947.