You could start here for its history and geography:
Student or Learner
I understand that the Cockney slang developed to obscure the meaning of the words used by his users. I am not going to learn it as I think it would be too time-consuming and rather useless for me, except for certain, most popular expressions which can be heard across the country.
What I would like to know is, whether the Cockney rhyming slang’s constructions are long- established and known to those born and raised or living within the sound of the bells of St. Mary-le-Bow church, or is still developing today with new words and phrases which can be made up like a pun/play on words and incorporated into that way of speaking, which would make the language even more difficult to understand by people from outside of the East End (and London in general) willing to learn it.
Yes, thank you, but what I mean is, whether it is still evolving, whether you can come up with some rhyming phrases and then use it, and still be understood by people speaking the same way in London.
It's not really a question of can; people do (and where they're born doesn't matter). For example, the expression 'I'm Hank Marvin' (meaning 'very hungry') can't have been coined before the Shadows' first hit (Hank Marvin was their lead guitarist) - in fact it must have been some time after that that he became a household name; and professional wrestlers' argot includes the expression 'doin' yer Gregory' - meaning 'pretend to have a neck injury'. These coinings came a long time after real Cockney criminals used 'Tom' ['...foolery/ jewelry'] or 'sausages ['... and mash/cash'] so as to keep their nefarious plans secret! (And you'll notice from that Wikipedia link that many people don't mention 'Cockney' at all.)