[Just asking] Is the SymmetricKeyAlgorithm.Plaintext not supported when unprotecting the private key?

[ghost]

I found that when the SymmetricKeyAlgorithm is plaintext, it will not derive the session key with the passphrase, and will raise a 'NotImplementedError'.

Here's the pgpdump result:

$ pgpdump keyfile
Old: Secret Key Packet(tag 5)(533 bytes)
	Ver 4 - new
	Public key creation time - xxxx
	Pub alg - RSA Encrypt or Sign(pub 1)
	RSA n(4096 bits) - ...
	RSA e(17 bits) - ...
	Sym alg - Plaintext or unencrypted data(sym 0)
	GnuPG gnu-dummy (s2k 1001)

...

It seems the plaintext is not supported, right? I wonder why. Whether it will support it in the future?