The underlying file format and associated libraries are probably largely unchanged from the NeXTStep era. If it ain’t broke, don’t fix it.

http://toastytech.com/guis/ns20mail.png

(Edit: beaten by 30 seconds!)

The dictionary app (and hence its database) may go all the way back to NeXTStep/Openstep.

Maybe not all of it. But I wouldn't doubt if bits of it went all the way back to the early days where a fancy dictionary was one of the star features of NeXTStep.