I use bits of backward music often...it's very easy in Reaper...record a chunk of audio. click on the item, select item properties or similar and select reverse...BAM...the image is reversed as well as the audio.
With spoken language it becomes more complex as a phoneme needs to convey/change meaning and a reversed phoneme may not, necessarily, do that.
These days there's a lot of "backmasking" done for fun & because the technology is so simple to acquire and use that experimentation occurs and many folk get a thrill from the "subversive" possibilities.
Revolution #9 has some but it's nothing particularly exciting. In fact it sounds much better forward than reversed...no shock in that.
Bowie used an Iggy Pop backing track in reverse for himself...he wrote the piece so that's fine.
I wouldn't call it "backmasking" if it's not actually masking something though.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe