Private Key is a 32 bytes data
Apply ECDSA or Elliptic Curve Digital Signature Algorithm to the private key.
The above step dervies the public key,.
(Legacy)Now append 0x04 to the start of the public key.
(Current Practise) Take X from derived public key. Now add byte 0x03 if the last byte of Y is odd or 0x02 if the last byte is even.
Now, apply SHA-256 to ...
Although hashing a public key by itself does provide quantum resistance, this is really only when it is considered by itself in a vacuum. Unfortunately, public key hashes do not exist in a vacuum and there are many other things in Bitcoin that need to be considered.
Firstly, if public keys were hashed, the funds are only protected before they are spent. As ...
The method described in the post has nothing to to with OP_RETURN, but is concerned with the SIGOP limits in a block.
Each block has a limit on the number of SIGOPs that can be present in transactions in that block. Signature validation is a CPU intensive operation, and the limit exists to ensure that no block gets too big to validate on regular hardware.