[KevinH]

FWIW, I thought Windows 11 fixed this by moving its default encoding to be utf-8 (Unicode 65001) for files and console use but maybe it was just Python on Windows that made that change.

Windows is alone in using an unnecessary BOM for utf-8 files as it breaks many many things on unix/linux/macos systems (ie try concatenating two text files and ending up with a BOM in the middle of the resulting file and breaking string compare, etc.) and it is against Unicode recommendations.

FWIW, the entire networked world has moved to default utf-8 without an extraneous BOM. IMHO, it is time Windows joined the networked world instead of adding tracking and advertising to its operating system. But that is just my opinion. Sigil will not support it except to strip it out.