Earlier quoted context omitted.
The motivation would be so that if you had a TrueCrypt archive lying around on a drive that you find in 5 years time, it would be possible to decrypt it - but they don't want to allow encryption because they won't be continuing development, and so fixing future bugs will not be possible.
If that's the motivation then in 5 years' time, who's to say the new version will work as well (assuming that it also won't be updated)? That doesn't make any sense.
At the same time, the developer clearly doesn't want people to be encrypting new archives with it, so have removed that functionality.
Removing the software in total would have lead to many mirror sites springing up, most with unproved providence (and a great opportunity for exploits). This method allows an official version to still exist (unmaintained, but still compatible with the current archives), whilst severely restricting new usages (by strongly warning against it and requiring using the dubious mirrors to obtain the software).