Soreness after exercise is not a prerequisite for muscle gains but may indicate muscles have been challenged.

“Do you need to be sore to make gains? No, I think that's pretty clear. But could being sore potentially be indicative that there may be some additional positive gains? It's not clear but a novel response, again, why does the body adapt? It adapts because it is being challenged beyond its present state. A novel response would generally indicate it's being taxed beyond its present state and, thus, having some degree of soreness might indicate that you are challenging it beyond its present state.”