Last time we saw of them, they were fighting against the Wakandans and the Avengers in Wakanda, and there was still a number of them.
Now, just after Thanos' snap, we see the Wakandans being dusted away, but not even a single outrider is in sight:
Now, it's pretty odd to not have even a single one of them in sight, when clearly the fight was still on and the battle was not over.
(...because if they had been defeated, all the other Wakandans, or at least M'baku, would've gone to aid the Avengers in their fight with Thanos)
What happened to them? Were they dusted? Did they retreat? Were they defeated?