because it appears those actually return from the enum UniMode, which is enum UniMode {uni8Bit=0, uniUTF8=1, uni16BE=2, uni16LE=3, uniCookie=4, uni7Bit=5, uni16BE_NoBOM=6, uni16LE_NoBOM=7, uniEnd}; according to the source code.
In N++ commit 8149f72 in v8.8.8 (Nov 2025), uniCookie has been renamed as uniUTF8_NoBOM inside the N++ source.
Thus, I just put in PythonScript Issue #421 to request that there’s a BUFFERENCODING.UTF8_NOBOM, either as replacement or alias of the old meaninglessly-named BUFFERENCODING.COOKIE … I don’t know if it will happen, but at least the request is in. :-)