From the original chat that got leaked, Walz had set the group chat up to auto delete messages every week, and 4 weeks in other cases. Official government communication is subject to the federal records act so that chat, which was used to communicate official government actions, was not compliant. If it was just used for lunch dates or something, Hesgeth wouldn't be in this mess.

CISA recommends Signal, but it is not an approved tool to share classified information. That's what SCIFs are for.