Does saying "Ya 'ayb ash-shoom" (Oh, what a disgrace!) upon witnessing a reprehensible act and feeling revulsion towards the perpetrator count as a forbidden form of divination (tatayyur)?

We do not know the intended meaning of the phrase "shoum," but it appears to be a borrowed term used to denote the ugliness of certain actions or words, and its speaker does not intend the forbidden pessimism. Such usage is permissible. The Quran described the inhabitants of Hell as "the Companions of the Left Hand" (Ashab Al-Mash'amah), meaning those afflicted with ill-omen and harm, as opposed to "the Companions of the Right Hand" (Ashab Al-Maymanah), meaning the people of good fortune and blessings. This is a common metaphorical expression in Arabic speech.