Since these are immutable we should just return a view of the original data rather than require a copy.

That could cause a memory leak by keeping the potentially huge and unwanted original array around.

The inverse is also true: this can cause massive amounts of useless objects by copying data that we know will never change.

True, this will be a much bigger change in that case, since we'll have to keep track of the length and offset on the underlying array manually.

I think there's advantages in re-using the underlying storage. For example, we read headers using a ByteString and it would be advantageous to do a zero-copy "split" it into name and value. Perhaps we document the fact that it's a view and require you to call .clone() if you want a defensive copy?

For what it's worth, I think the copy is less likely to surprise.