I think there is about no point in trying to convince “to subject or not”. The case has quite clear conditions.
Re: ... messages are required for mailing lists. But once implemented, the same visualization can also be very useful for direct in-reply-to answers in group chats: Referencing the original message even if just by showing a clickable default link “Re: First few words…”.
Otherwise, for chats, the subject is only relevant when sending non-encrypted messages, but this can still be integrated seamlessly and nicely.
The subject can be visualized and written in the editor completely unobtrusively for unencrypted messages, for example, with a slightly darker background. Together with visualizing the encryption state it may go like this:
🔓 When writing an unencrypted message, the text written until the max. subject length is then shown with a darker background while the typing can just go on normally beyond that. The resulting subject of the unencrypted message is then easily visible, and understood without requiring reading of documentation. And after seeing it in action when communicating with plain-email contacts, it will also become useful (for those who care) that the subject is easy to control:
🔓 A shorter subject
is very easy to “define” with this visualization scheme, if wanted, just by starting a new line before reaching the max. length of the subject.