I requested this back a few years ago and it was added in limited capacity.
I do have 40 column themes running on my BBS, but it does need some tweaking and you are correct the message editor does not run in 40, or I should say it does not display a 40 column mode, just continues. it's still workable.

My 40 column is text only.


SHIFT838
FuSiON BBS
fusion.zapto.org:9900

--- Mystic BBS v1.12 A43 2019/03/03 (Raspberry Pi/32)
* Origin: FuSiON BBS (21:4/164)

I would also like to have a 40 column theme to offer my TI-99/4A users.
Please let me know if there has been any progress on this. I would be happy
to help.

--- Mystic BBS v1.12 A45 2020/02/18 (Linux/64)
* Origin: ARDA-BBS (21:4/153)