"Mainframe" doesn't seem like an actual answer to me. Why can't mainframes handle passwords longer than 20 characters? Are mainframes incapable of doing passwor

"Mainframe" doesn't seem like an actual answer to me. Why can't mainframes handle passwords longer than 20 characters? Are mainframes incapable of doing password hashing? Can't modern databases have text fields with a max of 20 characters, why is that problem unique to mainframes?