All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon Prophet Muhammad and upon all of his family and companions.
A soul patch (also known as a mouche) is a single small patch of facial hair just below the lower lip and above the chin and it was never defined as a part of the beard neither in Arabic language nor in jurisprudence.
In this regard, it is stated in [Al-Qamoos Al-Muheet] what follows: "The beard is the hair which grows on person's cheeks and chin." Thus, soul patch hasn't been mentioned as a part of the beard, besides "Jurists considered the soul patch separated from the beard as stated in [Al-Majmooa'] whereas they added: "You should realize that are eight names/kinds of hair."
Accordingly, there is no harm on trimming, shaving part or all of the soul patch and the reprehensibility of shaving it which was stated by the Shafiee's doctrine issued at a specific time/era whereas the people who used to shave it were ascetics; therefore, Shafiee's jurists had great concerns about the people who pretend to be one of the ascetics and imitate them by shaving the soul patch for the sake of fame and glory. At that time, it was stated by Al-Khateeb Ashrbini:" Shaving or trimming the sides of the soul patch for the purpose of asceticism manifestation is reprehensible." [Mughni Al-Muhtaj vol. 18 pp. 169].
As for the time being, the reprehensibility of shaving or trimming it for the above purpose don't exist within people's minds, thus there is no single reason to reprehend this matter. And Allah Knows Best.