https://www.physicsforums.com/threads/so-32-heterotic-string-theory.829219/

suggests that the reasoning which leads to SO(32) open string in ten dimensions (as a way to cancel gauge and gravity anomalies), can be repeated for specific compactifications.

Meanwhile, I always thought leptons and the electroweak sector might emerge in the IR, via a Seiberg duality with a UV theory which is just quark and gluon superfields. The emergent composite fields in the IR of a Seiberg duality must obey some kind of anomaly matching.

Actually, I’m reminded of a «Hanany-Witten transition», in which new branes emerge at a brane intersection, which has been a string model of some Seiberg dualities. It would be interesting if emergence of an electroweak sector could be understood that way.