That's honestly "common". I've seen it a number of times emails along the line of "let's not talk about this via email but let's discuss it only over the phone" - in many other companies/situations.
Sure. In the context of an already-filed lawsuit, however, that's asking for trouble. The other side has the legal right to discover relevant documents that you hold; deliberately trying to make that harder during a court case is arguably unethical, and is certainly frowned on by judges.