> "UTF-8" support was often just single byte, so it was little more than ASCII

"Single byte UTF-8" is ASCII. That's one of its most important properties.

> Even now it's common for programs to choke on the optional BOM

And they should... BOMs (and especially the hilarious UTF-8 BOM) are strictly a legacy Microsoft/Windows thing and should be abolished along with "extended" 8-bit ASCII encodings and UCS-2/UTF-16 (only UTF-32 makes sense, but should only be used at runtime to allow random access on UNICODE code points, but not for data exchange.