I don't *think* it's *explicitly* illegal, i.e., no law to fit the offense. There are lots of autopen-signed documents (of lesser importance, and by several other presidents) out there.
NEW FACTS MAY ERASE JOE BIDENS TERM IN OFFICE.
HE NEVER SIGNED ANY OFFICIAL DOCUMENTS... INSTEAD,
AN AUTOPEN SIGNED ALL BUT ONE DOCUMENT,
HIS LETTER ANNOUNCING HE WOULDN’T SEEK REELECTION.