It is case insensitive by default, but you can't guarantee that every volume mounted on a Mac is case insensitive.

Use diskutil to see if a volume is case-sensitive or not:

$ uname
Darwin
$ diskutil info disk0s2 | grep 'File System'
   File System:              Journaled HFS+
$ diskutil info disk1s2 | grep 'File System'
   File System:              Case-sensitive Journaled HFS+
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