Together with the question you are linking, there is another related question in Unix & Linux: Are there naming conventions for variables in shell scripts?.
There you can find a couple of good answers:
Variables that are introduced by the operating system or start up
scripts etc. are usually all in CAPITALS, these are called
'envrironment variables'.
To prevent your own variables from conflicting with environment
variables, it is a good practice to use lower case.
Together with a Shell Style Guide link, where you can find:
Naming Conventions
Function Names
▶ Lower-case, with underscores to separate words. Separate libraries
with ::. Parentheses are required after the function name. The keyword
function is optional, but must be used consistently throughout a
project.
Variable Names
▶ As for function names.
Constants and Environment Variable Names
▶ All caps, separated with underscores, declared at the top of the
file.
There is no suggested convention in man bash, just note the "be careful with uppercase" warning.