Hi, for your info, this convention is specified by POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html
Which says:
Environment variable names used by the utilities in the Shell and
Utilities volume of POSIX.1-2017 consist solely of uppercase letters,
digits, and the <underscore> ( '_' ) from the characters defined in
Portable Character Set and do not begin with a digit.
And:
The name space of environment variable names containing lowercase letters
is reserved for applications. Applications can define any environment
variables with names from this name space without modifying the behavior
of the standard utilities.