Is there a barbarian encampment spawn cut-off trigger?

My pangea map has a lot of free space for barbarian encampments to spawn. I had been farming them for cash and culture.

However, I am now in the 1700s and notice that they are not spawning anymore. Is there a technology I have discovered or have I passed a time line which stops the encampments spawning?

No. Barbarians should continue to spawn as long as there is area that is not within the cultural boundaries of any civ, nor within sight of any units. You state that there is lots of free space for barbarians, but how do you know this? Maybe posting a screenshot of the map with the areas where you thinks barbarians should be spawning would help clarify the situation.