The Jargon File (version 4.4.7, 29 Dec 2003):

Great Runes
 n.

    Uppercase-only text or display messages. Some archaic operating systems
    still emit these. See also runes, smash case, fold case.

    There is a widespread legend (repeated by earlier versions of this entry,
    though tagged as folklore) that the uppercase-only support of various old
    character codes and I/O equipment was chosen by a religious person in a
    position of power at the Teletype Company because supporting both upper and
    lower cases was too expensive and supporting lower case only would have
    made it impossible to spell ?God? correctly. Not true; the upper-case
    interpretation of teleprinter codes was well established by 1870, long
    before Teletype was even founded.


The Free On-line Dictionary of Computing (30 December 2018):

Great Runes

   Uppercase-only text or display messages.  Some archaic
   operating systems still emit these.

   See also runes, smash case, fold case.

   Decades ago, back in the days when it was the sole supplier of
   long-distance hardcopy transmittal devices, the Teletype
   Corporation was faced with a major design choice.  To shorten
   code lengths and cut complexity in the printing mechanism, it
   had been decided that teletypes would use a monocase font,
   either ALL UPPER or all lower.  The Question Of The Day was
   therefore, which one to choose.  A study was conducted on
   readability under various conditions of bad ribbon, worn print
   hammers, etc.  Lowercase won; it is less dense and has more
   distinctive letterforms, and is thus much easier to read both
   under ideal conditions and when the letters are mangled or
   partly obscured.  The results were filtered up through
   management.  The chairman of Teletype killed the proposal
   because it failed one incredibly important criterion:

     "It would be impossible to spell the name of the Deity
     correctly."

   In this way (or so, at least, hacker folklore has it)
   superstition triumphed over utility.  Teletypes were the major
   input devices on most early computers, and terminal
   manufacturers looking for corners to cut naturally followed
   suit until well into the 1970s.  Thus, that one bad call stuck
   us with Great Runes for thirty years.

   (1994-12-02)