Missing storage format:
32 bytes of raw binary.
Since random data is incompressible, it even remains unchanged if your wallet is in an (unencrypted) zip file.
If your wallet is on an old hard drive or USB stick, back up and scan a raw image of the ENTIRE DRIVE. Even if your wallet is encrypted or damaged, you might just find your private key in a chunk of an invisible cache or page file somewhere else on the disk!
As a last resort, I made a highly optimized tool to exhaustively scan a raw disk image for a binary private key, using a sliding window of 32 bytes. It still took *months* to scan a big hard drive, though.
To be sure, I recommend making a disk image using ddrescue with SystemRescueCd, then make a backup of that image, then scan the image.
Another missing storage format:
ASN.1 DER.
Early versions of Bitcoin used OpenSSL and its related overengineered storage standards. It might be binary, as above, with various leading bytes, or it might be a base64 pem file (with newlines in the middle of it).
References:
https://github.com/dsharhon/bitcoinj-minimal/blob/master/core/ECKey.java#L66https://en.wikipedia.org/wiki/ASN.1#Example_encoded_in_DERhttps://www.hanselman.com/blog/DecodingAnSSHKeyFromPEMToBASE64ToHEXToASN1ToPrimeDecimalNumbers.aspx