@hypolite I understand. It is not easy as there are so many connection types.
Best practice would be to handle "specialities" inside the connector addons. I know that AP services don't need the subject and interpret them as CW when found (tested mastodon, pleroma, hubzilla), as DMs are normal posts to mentioned accounts only normally.
How diaspora or twitter handles them, if they need or use them at all, I can not say of course. But would be cool to not have content put to the post in general, only when really needed (for that: in connector addons).

But I understand of course, that you core devs really have no time to do all that. Maybe "between the years" (like we Germans say) is time for me to install a dev environment for core source.. :)