Well traditionally they keep the stretching instruments in because it is part of their decoration and culture, but if they stop due to adopting "modern" ways the results may be permanent.

There is plenty of anecdotal evidence that PE gains can be permanent. Some report losing some but not all gains, others report not losing any at all. I myself did jelqing and other PE exercises four to five years ago but then stopped until recently and in that time did not lose any of the 3/4" gain in length I had.

There is no scientific evidence for PE as far as I know, as it seems no research has ever been done into it - it probably would not be considered respectable to publish in a scientific journal.
However the articleHeat and stretch procedures: an evaluation using rat tail tendon from the Archives of Physical Medicine & Rehabilitation
does provide evidence of permanent stretch in animal tissues. Heat and stretch procedures: an evaluation using r...[Arch Phys Med Rehabil. 1976] - PubMed Result There are also other articles in scientific journals into the permanent stretching of connective tissues etc.