Mixup
Now let's check how things have improved with 10.5 Leopard. -o List in long format, but omit the group id. Somebody had a good idea. Why would anyone using a long format listing not want to see the group ownership?
What struck me about the -o flag is that this behavior is listed in the Linux manpage.
-o like -l, but do not list group information
Mac OS X is supposed to have BSD running underneath, where the -o flag is used to display something completely different.
-o Include the file flags in a long (-l) output.
Expecting -o to give you file flags and receiving long format without group names instead would be quite a shock to a developer.
With more detective work, the reason for the change in the -o flag involves Mac OS X Leopard’s certification to the UNIX ‘03 specifications.
Leopard is an Open Brand UNIX 03 Registered Product, conforming to the SUSv3 and POSIX 1003.1 specifications for the C API, Shell Utilities, and Threads. Since Leopard can compile and run all your existing UNIX code, you can deploy it in environments that demand full conformance — complete with hooks to maintain compatibility with existing software.
The UNIX ‘03 specification states that the -o flag will
-o The same as -l, except that the group shall not be written.
Which seems explain the change.