The same could be true of the other piece of Boba Fett’s original outfit that is missing in The Mandalorian.
Like the cape, the braid likely simply didn’t survive the wear and tear of the years the way the armor would.
Fett had a braid of some material that wrapped around the right shoulder of his armor which is no longer there.
At one point in Star Wars history, it was suggested the braid was made of Wookie scalps that Fett had collected from victims, but that description predated even the prequel trilogy, so it’s possible the “canon” source is something else entirely.