In this thread i posed a related question, and also arrived at the conclusion that we need a non-mutating version of deleteat!. That thread goes into quite a lot of detail showing how users might be tempted to implement this themselves, which to me showcases the “danger” of not providing this functionality out of the box.
While I agree that it is weird to have a non-mutating version of deleteat!, just conceptually, there is no argument against it’s usefullness, which I think should come first. Especially given Julia’s strong conventions around using a bang to indicate mutation.