The concept of Backmasking has been well and truly debunked. It was popular for a while in the eighties but fell out of fashion completely when everyone realised it was rampant nonsense.
There are bands who have deliberately recorded music forward and then put it backwards on albums or singles, most notably the Beatles. This is clearly apparent when the record is played backwards and there is no ambiguity. The recording also makes no sense when played forwards.
There are other bands who have been accused of including satanic messages in their songs which are sound like normal vocals when played forward but sound like satanic messages when played backwards. This was done (the theory goes) because the brain can interpret these messages subliminally so while the listener thinks they're hearing Robert Plant sing about a lady who's sure, the brain is really registering a message about what a great guy Satan is.
This caught on in the eighties amongst religious fundamentalists in America who didn't have much to do in between prayer meetings so sat around with record collections playing them backwards and getting upset about the presence of the devil.
The issue was legally resolved in 1990 when Judas Priest were sued by the families of two teens who killed themselves not long after listening to one of their albums. The families alleged that Judas Priest included messages including the repeated phrase "do it" and other suggestions that the listener should commit suicide. The case was thrown out when Judas Priest proved how silly the whole thing was. The band spend an afternoon listening to the entire album backwards and writing down messages that they thought they heard. The next day they went into court and asked the lawyers for their accuses to play their own copy of the record backwards. The lawyers for Judas Priest were able to say: in the next track you will hear the words "I want a peppermint get me one" and other nonsense phrases. The judge had to admit that he could clearly hear every word they pointed out.
Judas Priest conclusively proved that the sounds the human voice makes when played backwards are like Rorschach inkblots: we hear what we want to hear. If we're looking for Satanic messages we find them, if we're looking for nonsense we hear that too. Once you're told you can hear something you can clearly hear it thanks to the power of suggestion.
The other damning evidence was the fact that the judge failed to find any scientific evidence to support the idea that the brain can actually interpret a backwards message whether intentional or not. None of the consulting experts could back up the idea that subliminal backwards messaging gets stuck in the brain at all. Judas Priest were also quick to point out that they had nothing to gain by having their fans commit suicide and that if they could include powerful subliminal messages in music they would have included the words "Buy more Judas Priest albums"
In the case of Stairway to Heaven the accusations have been denied repeatedly by everyone associated with the making of the album. They were denied when they first came up and they're still denied today in an era when hardly anyone would care if they said "Yeah we put satanic messages in our music back then".
It's nonsense without a single shred of truth.EDIT:
I didn't feel the need to address Stairway specifically when I wrote this because I assumed nobody really took the nonsense seriously. But I've since had discussions with people who seem to believe that "evidence" like the fact that Led Zeppelin spent some time in a cottage in Wales and at one point wore some strange clothes means Stairway must be Satanic. So I've felt the need to say a few more things.
The alleged message that is clearly discernible in Stairway to Heaven is as follows..."Here's to my sweet Satan/The one whose little path would make me sad, whose power is Satan/He will give those with him 666/There was a little toolshed where he made us suffer, sad Satan."
Let's unpack that a little shall we?"Here's to my sweet Satan"
-Fair enough. If I was a hard core Satanist and I was going to put a satanic message in a song then that's probably something I'd throw in."The one whose little path would make me sad"
-Hang on. If I'm a happy little Satanist who thinks Satan is sweet then why is he making me sad? Surely Satan would make me happy what with me being a devil worshipper and all. And why is his path little? Who describes anything as a little path?"He will give those with him 666"
-He's giving 666 now? What does it mean to be given 666 exactly? "There was a little toolshed where he made us suffer"
-A little toolshed where Satan made you suffer? Okay ignoring the fact that referencing a little toolsheds is kind of bizarre, why did Satan make you suffer? And if he did why are you following him? "Sad Satan"
-So Led Zep are sad on the little path and Satan is sad as well. Why is everyone so damn gloomy? How is this promoting devil worship? Who wants to sign up for Satanism if everyone's so miserable all the time?
So the message that allegedly promotes Satanism is actually saying that if you follow the Devil he'll lead you up a sad little path into a toolshed where he'll make you suffer but he won't be happy about it. That makes no fucking sense.
If the accusation is that the normal, forward playing lyrics to Stairway to Heaven are total nonsense so it makes sense as a Satanic message when played in reverse then why doesn't it make more sense when played backwards? Why not write a more coherent message which actually makes Satanism look more promising than just a lot of morose unpleasantness in an undersized hardware storage facility?
The real test (and I've done this) is to play the part backwards to a third party and ask them what they hear without giving them any prior knowledge. I've played it to two separate people and asked what they heard. Neither could come up with anything really coherent and nothing like what the message purports to be. When I told them if they could hear the message that's reputed to be there (i read it to them) they could suddenly make parts of it out. So what kind of message is it if people can't understand it unless they're reading it at the same time? A real message or just a fictional one that relies on the power of suggestion to remain alive?
If you listen to the song played backwards (and it's all over youtube) you will realise it only works if you ignore certain syllables. There are vowel and consonant sounds which have to be totally disregarded for the effect to come close to working. As a message it requires your own personal mental editing to actually work in the first place.
Finally I think it's worth pointing out that Robert Plant has not only denied this constantly over the last 40 years but he's actually pointed out that he finds the suggestion kind of offensive. Without any evidence at all and using totally debunked theories, people are happy to accuse a guy of being in league with the Devil. Whether or not you believe in Satan and his toolshed there's no doubt that Satanism is not really something that most people would be happy to be accused of. It's got associations with ritual sacrifice and other nasty stuff and it's a pretty hurtful thing to level at a guy who has always been big on notions like peace and love and is definitely not the type to participate in the sacrifice of virgins. People might like to consider this when they make unfounded accusations: you're calling Robert Plant a Devil Worshipper and he's politely asked if you could stop.