It doesn’t feel like a feature to me. And neither does the lack of deniability. They both feel like things that leak information that doesn’t need to be leaked.
Perfect forward secrecy requires two-way real-time communication, in order to construct a session key that can't be computed from just the private keys and the encrypted message. Therefore the way that PGP's lack of perfect forward secrecy is a feature is that it allows an encrypted message to be generated in a way that doesn't require two-way real-time communication, and can therefore be sent by email.
The trade-off is that you then don't have perfect forward secrecy.
Seems somewhat like threat model will determine the need for deniability etc. I don't consider myself to need it, and mine seems like a common enough case - compatible with a normal WhatsApp user's use case.
