To exemplify the question I'll create a scenario.
The enemy team has a Reinhardt and Mei that are being particularly troublesome, we have a tracer that flanks around the rear of Reinhardt and sees the enemy Mei, the tracer decides to target the Mei as she has the ability to slow down the tracer.
In this scenario, could the friendly tracer shoot through the enemy Reinhardt's shield (from his side of the shield) to then hit Mei?
Best Answer
No. Shields absorb enemy projectiles from both sides. Likewise, allies can fire through it from both sides. That is, the team the shield belongs to determines whether projectiles pass through it.
My only source is primary: approximately 150 hours of play, much of it as Reinhardt.