For purposes of immigration removal, the question of whether a particular group is a cognizable social group under the statute is a question of law.
Former taxi drivers who witnessed a murder and were subsequently threatened by a gang do not constitute such a group.
http://media.ca8.uscourts.gov/opndir/18/06/171430P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.