> How official is that?

https://github.com/jythontools/jython
jeff5 is now maintaining Jython2 but I can not find Jython3 under development.

> Does it cost any maintenance burden? If not, you have your answer :-)

Yes, it currently not burdens for maintenance.
But is the policy of CPython to care about alternative interpreter implementation? So if the new alternative implementation request to modify CPython code. Is it okay?
If so I agree with not to remove Jython code on Python3.x

But if not, can we gradually remove about alternative interpreter related code? Should we preserve the area for Jython?
https://github.com/python/cpython/blob/master/Doc/using/cmdline.rst#options-you-shouldnt-use

Okay, I understand that there must be a historical reason for those codes. But can we move a step for deprecating them?