Have all hashes of unlocked difficulty levels of cryptocurrencies 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).