I guess I found the reason... maybe obvious but it wasn't for me.
The forum "permissions" and the group/user "privileges" should make sense to create a "type of permission" option.
In other words, if the forum is open to all/registered users, it doesn't make sense to show which permission you want to allow/restrict.
If the forum is "private", then the permission option will show.
I hope this helps others too
Many Thanks to all Open Source Community

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