The problem here is less that MARC21 is a standard from 2000, and more that it’s a compatible extension of MARC, which is a standard from ~1970. At that vintage it’s lucky that they were hip enough to specify ASCII (which was still awfully new), and downright miraculous that it ended up 8-bit clean (field names are restricted to printable ASCII, and framing is a combination of C0 control codes and printable ASCII, but field values are whatever you want).
The problem here is less that MARC21 is a standard from 2000, and more that it’s a compatible extension of MARC, which is a standard from ~1970. At that vintage it’s lucky that they were hip enough to specify ASCII (which was still awfully new), and downright miraculous that it ended up 8-bit clean (field names are restricted to printable ASCII, and framing is a combination of C0 control codes and printable ASCII, but field values are whatever you want).