Thanks for the suggestion.
Yes, we’ve recently migrated from a mailing list from the 1990s. Ironically, the previous mailing list also had “reply” go to the entire mailing list, so they are requesting a change in behavior.
Users can have some peculiar requests. In this case, I agree with the user that Discourse should follow email conventions when interacting via email. I made a similar feature request to Discourse Meta for stripping “Fwd:” when creating a topic based on a forwarded message: users won’t - and shouldn’t have to - break email convention when forwarding an email to the forum.