Muttar or assur??
Well, in the shiur I referred to where the Shlissel Challah was banned, red strings similarly got the axe. There is an explicit Tosefta [body of literature dating back to the time of the Mishna] forbidding it as a goyishe, superstitious practice. Clear cut. Assur. Nicht voos tzu redden. Nothing to talk about.
Not so clear cut. In that same Tosefta [Shabbos 8/4] there is the opinion of Rabban Gamliel permitting it. A survey of the poskim will reveal that many not only permit the practice but actually encourage it as a segulah against bad things [see the sefer Lvush Malchus page 72 of R' Menachem Giat for the many sources].
On my wrist you will not find a red string. You will find a watch which reminds me with every movement that I am closer to my demise and that time is RUNNING OUT. Nor have I ever seen a talmid chochom wearing one. BUT that doesn't mean it is forbidden.
I would rather assume that people aren't sinners. I mean no disrespect chas vi-shalom to the Rav with whom I am taking issue. Just seeking the truth.
