Rhyming slang rhymes tho. The recipient can understand what's meant by de-obfuscating in-context. Random strings substituted for $proscribed_word don't work in the same way.
In Cockney rhyming slang, the rhyming word (which would be easy to reverse engineer) is omitted. So if "stairs" is rhyme-paired with "apples and pears" and then people just use the word "apples" in place of "stairs". "Pears" is omitted in common use so you can't just reverse the rhyme.
The example photo on Wikipedia includes the rhyming words but that's not how it would be used IRL.
