On the UK Pepper it had a blurb (released on Anthology) that was a mishmash of sounds and voices. It is about 2 seconds and was put into the record's runout groove so it would repeat forever or until you or the turntable lifted the tonearm.
It did NOT say 'f*** you like superman', in fact it doesn't 'say' anything. Many people have heard many different things because it was a collection of noises and sounds as noted above.
Backwards? All of Rev #9. Horrendous car crash, Paul yelling, sirens, people running and yelling, turn me on dead man...it is all there. On purpose? Beats me, I don't believe Paul is dead (yet). But it is pretty spooky to listen to.
Between I'm So Tired and Blackbird is John speaking (recorded backwards) and it *sounds* like 'Paul is dead man miss him miss him miss him.' Primarily because that's what we've been told to hear so the brain agrees. It IS close tho, closer than the 'superman' hype.
There's more of course, but not much of any real consequence.