Is it safe to build these bookkeeping tools and put client bank statements into the free version of Claude or ChatGPT?

No — the group's guidance was clear that you should not put client bank statements or any client-related data into the free version of Claude (or presumably ChatGPT), because free-tier conversations are used for model training. Paid versions (Pro and up) do not use conversations for training and offer encryption in transit and at rest, but you still need to check settings to make sure training is turned off. This is described as more of a security/data-security issue rather than a 7216 disclosure issue, which is treated as a separate concern.