There are solutions for the yEnc encoding in FossaMail, like yproxy (converts yEnc to UUE on the fly). I think yproxy may even combine split posts as well, but I'm not sure.
I won't add yEnc to FossaMail because it is not RFC compliant, and requires news servers to also operate outside of the usenet spec to transport yEnc encoded messages.
FossaMail (and Thunderbird) are simply not binary news readers. They are readers meant for using usenet in its originally intended way: public, distributed discussions on various topics. That is the core of usenet in the client, and that is not something to consider removal of even if using the net as a free binary sharing portal that way has become more popular.
If you want to work with binaries newsgroups, then you should use a dedicated reader for it - or a third-party program to work with FossaMail.
EDIT: I just looked at the official yEnc home page (and some related articles) and the average age of all that material is 10 years or more.
So, anything remotely useful is at the very least completely bitrotted, and likely not even applicable to FossaMail's supported operating systems...

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