Just a bit of reading between the lines:
the Thunderbird team has decided to take a different approach
and
The MZLA team is working to essentially rebuild the software from scratch, featuring a new interface and support for modern email capabilities.
I may be over-reading though.
This original was already a privacy nightmare, even on GOG, which is why I was hesitant to buy it. Looks like I’m definitely pirating it now if I wanna play it. But hey, at least now we know why sony insisted on “remastering” it.