Interesting! I made a very similar tool earlier this year.
It comes with presets for various different areas of Unicode, and some example text, although the intended use case was very different, I looked at it from a steganography perspective rather than an honours-system obfuscation perspective.
I initially thought it would be able to decode the rot8000 output without any modification but I think the utf-8 escaping that my tool expects (from its own output) gets confused by the output from rot8000.
It may also be that you're rotating by 0x8000 and this code is not. It's creating a mapping that's restricted to non-control, non-surrogate, non-whitespace characters and rotating by half the size of that mapping.
This will break, i.e. two consecutive rotations will no longer be the identity, if the number of valid characters in the BMP ever becomes odd. And there are still a few unallocated code points in the BMP. There is also an overflow in line 39 because of the check i <= BMP_SIZE in line 37 which, I guess, previously used Char.MaxValue instead of BMP_SIZE. But it does no harm here, U+0000 just gets filtered out twice.
It comes with presets for various different areas of Unicode, and some example text, although the intended use case was very different, I looked at it from a steganography perspective rather than an honours-system obfuscation perspective.
https://incoherency.co.uk/mojibake/
I initially thought it would be able to decode the rot8000 output without any modification but I think the utf-8 escaping that my tool expects (from its own output) gets confused by the output from rot8000.