I developed some code using wiringpi on a 4.4.21 machine (Jessie) - it worked fine and as expected, required root privilege to run (i.e. requires sudo) - it uses wiringPiSetup().
I moved the code over to a 4.9.59 (Stretch) machine and re-compiled - but when run it doesn't require root privilege (i.e. sudo not needed). Both are being initiated by the "pi" user. The Jessie machine is V1.2 pi zero (no camera connector). The Stretch machine is a V1.3 pi zero (with camera connector - but not a W).
The only other difference I've noted is that during compiling/linking on Stretch it required the inclusion of unistd.h for the use of sleep() which it didn't seem to need on Jessie (least ways cc didn't complain).
Why is root not required on Stretch, but it is on Jessie? I did not expect this to be the case.