Deniability and perfect forward secrecy are at odds with how people use email anyway. But that doesn't stop people from demanding both very loudly, and some people from promising them.
I don't know what you're saying. Most email isn't encrypted so PFS wouldn't apply to the content of email messages. When you mention PFS are you talking about email encrypted with something like GPG/PGP? Or are you talking about PFS in the context of the TLS connections used to protect SMTP? I'm not sure what "deniability" refers to in your post and how you think people use email in ways that would contraindicate it. Once I understand what you're saying, I guess we could move on to who the demanders and promisers are and what you think they're doing.
Deniability and perfect forward secrecy are at odds with how people use email anyway. But that doesn't stop people from demanding both very loudly, and some people from promising them.