I think there are two things you may be referring to. First, some drives support thin provisioning of namespaces, and can report the current amount of storage actually being used to back the namespace(s), relative to the maximum user-accessible capacity.

Second, in the SMART/drive health data, there's usually a counter tracking how many reserve/spare blocks remain usable as in not worn out and retired. That's a one-way counter; deleting data won't un-retire defective or worn-out blocks.

It's almost unheard-of for a drive to directly expose a realtime counter of how many blocks are currently unallocated, not being used to store data, and in the erased state making them available to accept new writes.