I've seen this thrown around as the solution before, but it actually doesn't work. The official SSH Key type uses special fields for the Public Key and Private Key fields that looks like single-line input boxes but are actually multi-line textareas. Custom Templates don't have access to those field types and cannot store ssh keys correctly.
Easily copying the private key is a terrible idea from a security perspective, while easily copying the public key and password are by far the most common use cases for stored SSH keys. That's why threads like these keep showing up.
Seriously, just fix it.