Once back in 2014, theymos received a subpoena for the DPR case to release the deleted posts and PMs (though not really sure of the latter).
Theymos can access users PMs but like as others mentioned, he will not check PMs until users have done something extreme. I have read somewhere theymos to say something like, "I don't read anyone's PM unless it is necessary. I have once provided some PMs for the requirement of FBI (Or something same) for an investigation."
I can't find that post though.
this is one, searching the other one-
I don't read PMs for fun. Very very rarely, I need to read PMs for technical, administrative, or legal reasons, but I try to avoid this as much as possible. I don't want to spy on anyone's personal business.
Update- Didn't find the one I am searching

this post?:
Nah, it was close enough what I said.