Over in in /r/worldnews there is an article reporting that this treaty exists.
http://www.reddit.com/r/worldnews/comments/20qpy7/turkey_under_ottoman_empire_treaty_with_catherine/
It gives no name, just that it was signed 230 years ago. If it exists, is it in anyway enforceable or is it merely a curiosity?
That is absolutely not right. It's the treaty of Küçük Kaynarca, signed at the end of the Russo-Turkish war which is salient to this point.
So, to be clear, the article doesn't even have the date correct. 1783 is not the date of the treaty that establishes the Crimea of an independent nation. Quite the opposite, actually.
Among its many stipulations and their implications is the creation of a complicated relationship between the Crimean Khanate and Russia. The Crimea had mostly been, since the Mongol invasion, an independent nation. After the Russo-Turkish wars, several ports were ceded to Russia on the Black Sea, as well as sweeping protections for Orthodox Christians in the Ottoman Empire and the Black Sea region in general.
However, the Khans were thereafter extremely dependent on Russia due to the terms of the treaty, and in 1783, Catherine outright annexes the Crimea.
I have not read the treaty in full, so I am not certain whether such a clause exists. But as they often do, /worldnews has upvoted something nonsensical without doing the research themselves.
Whether or not the clause exists, the Khanate did declare independence following that war, and the treaty specifically has both sides acknowledge the sovereignty of the Khanate. To me, it makes no sense whatsoever that such a clause would exist, because the terms of the treaty obviously castrate the Khanate politically and make it ripe for Russian annexation. The Khans have no reason not to simply return to the bosom of the Ottomans.
The treaty would not apply in any sense here, because it only refers to the specific sovereignty of the Tatar Khans in Crimea following the Russo-Turkish War. People in general do not understand how treaties work, and I'm not certain why. The '74 treaty established the Khanate as an independent state. The '83 annexation ends that independent state, making references in the treaty to the Crimea obsolete forever after.
The treaty that you are talking about is called the Treaty of Küçük Kaynarca.
I brought the same question up in the Crimea megathread two weeks ago when I read another article, that didn't quite get the /r/worldnews 'coverage' this one did, with replies from two flaired users.