Think of it as a single, shorted secondary turn but *outside* the
laminations. Any field that leaks out of the transformer induces a
current into the strap, and it in turn generates a field that fights
the excitation. It should be thick, wide, and well-shorted, which
means well soldered. Copper is best.
That is a completely garbage overstatement. Just the same, any tight bend
(bending r less than 10 thicknesses) will require annealing in a reducing
atmosphere.
It is really quite intolerant of cold working if you wish to maintain the
magnetic properties. Do not even flex it unnecessarily.