Since post authors expect to be able to view the passwords they assign to posts (and often don’t write them down), it will continue to be stored in plaintext.

Wait. What?

Slacking so blatantly on security only because of convenience is scary when it's part of a platform powering 25% of the web.

I'd never use post passwords myself as the idea seems counter-intuitive.
The author would have to share the password to everyone he'd want to give access to the post, and he'd most likely do it over an insecure channel. If the password hasn't been eavesdropped by then, it only takes 1 naive or malicious user to create a breach.