The best known of the arguments that Cyrillic drew on a Semitic script rests on this letter. Sha and the Hebrew shin write the same sound and look, to many eyes, similar: three uprights rising from a base. The comparison has been made since the nineteenth century and it has never gone away.
The case for it is not only visual. Whoever assembled the alphabet was working in a milieu where Hebrew was not unknown, and the Slavonic accounts of Constantine credit him with an interest in other scripts and with disputation involving Jewish scholars. A scholar who needed a sign for a sound Greek lacked, and who knew an alphabet that had one, might reasonably take it.
The case against is that the argument proves too little. A shape made of three strokes on a base is one of the simplest a pen can produce, and simple shapes coincide. Glagolitic already had a letter for sh, and a maker of Cyrillic had every reason to use the sign already fitted to the language rather than reach further. And the Semitic hypothesis, extended to the other non-Greek letters, produces matches of steadily worse quality, which is what one expects from a method that finds resemblances rather than descent.
What would settle it is evidence of a kind nobody has: an exemplar from the period, or a statement by someone who was there. Both camps are working from later manuscripts and from shape comparison. The atlas therefore records the derivation as debated rather than picking the side with the better story.