From the beginning, there were only functions named used_in_bytes() and capacity_in_bytes(). These functions iterated over the ClassLoaderDataGraph to reach all metaspaces and then iterated over the chunk list.
Later, the functions allocated_used_bytes() and allocated_capacity_bytes() were added that used accounting to be able to return a value faster and without using a lock. When these were added, the functions used_bytes() and capacity_bytes() were renamed to used_bytes_slow() to capacity_bytes_slow() to clarify the difference (the slow functions were later made private).
Since the old, slow functions end in _slow, we can now drop the allocated_ prefix from the accounting functions.