Yes, these are the normal semantics, even on modern systems. If you want to prevent a user from removing a file, remove the user's write permission to the directory containing the file. Recall that a "file" in the removable sense is really a directory entry that points to the inode that represents the real file; to remove that, one must modify the directory to remove the directory entry. The permissions on the file itself don't matter since removal isn't an operation on the contents of the file; the only thing it does to the actual file is update the link count in the inode.

        - Dan C.