I do see evidence that Lightning Labs sometimes does batched channel opens via their Lightning Pool service: "All cleared orders in a batch are executed using a single batch transaction, allowing participants to pay lower chain fees compared to non-batched channel opening." https://lightning.engineering/posts/2020-11-02-pool-deep-dive/
Lightning Pool is different from Lightning Loop, but it's at least an example. I don't think Acinq has ever said they do batched channel opens for their users, but I might be missing something.
