Have all hashes of unlocked difficulty levels of cryptocurrencies been mined? I'm wondering if all the hashes of a previously unlocked difficulty level of a cryptocurrency, such as 


  0000000000000000xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx BTC


, have been completely mined, where x is a placeholder for a constituting HEX digit, such that all the possible combinations of digits in that difficulty level have been mined.
No, and this will not happen in the foreseeable future.

Any given hash calculation will result in any one of 2^256 possible numbers. That means, the odds of finding any one exact number is 1/2^256.

Even at today's hashrate of ~47000000 TH/s, it would take you 7.812×10^49 years to find a preimage for every one of those 2^256 numbers, assuming each hash you find is unique (no collisions).