In late 1977/early 1978 I used a Honeywell Level 66 mainframe whose timesharing interface was lower-case. My memory is hazy, but I’m pretty sure I typed in the FORTRAN in lower case, and the printouts were in upper case, including the comments. Probably, as mentioned by other posters, the line printer only had an upper case train. Certainly I typed lower case into the command prompt.

The operating system on that piece of overworked big iron was GCOS, formerly GECOS. This is the very same GECOS that’s still a field in /etc/passwd, where it was originally used to store the user’s GECOS account name. Bell Labs had a GECOS system and you could spool jobs to it from the Unix systems, so that was why the field existed. It shows the Unix creators were familiar with that operating system.

In 1977 Unix Version 7 had only just come out and was beginning to appear outside Bell Labs. Its influence on the wider computing world would have been slight.

However, I doubt the Unix creators got the lower case directly from GECOS. They had worked on Multics, which was also lower case. (Coincidentally, Multics ran on the same hardware as GECOS, with the addition of a memory management unit.)