Why not just have an easy button that you can click saying Do Not Allow Reply All?
I know that there are some ways you can limit reply-all availability, like in the URL linked here. But there’s a note: If recipients open this email in other mail applications except Microsoft Outlook, such as opening on web page via web mailbox, they can reply all this email.
I’m semi-tech savvy but I’m no programmer. It feels like it should be easy to do, so either I’m totally wrong or email services are really missing out on a great thing they could do.
It is slightly the fault of the email clients for the sender that often don’t show BCC by default. It probably would be reasonable for email clients to put a warning up if people are sending to a large number of people without using BCC.
Also their fault because a lot of them have had the Reply All button first before Reply. Outlook, at least, seems to be changing this in some ways.
But putting it first is guaranteeing users will just click the first “reply” and keep writing.