Unmodified and stock ROMs for the Samsung Galaxy Tab S2, specifically SM-T710, SM-T810 and SM-T815 are available for download. The stock ROMs are based on Android 5.0.2 and Android 5.1.1.
These are unmodified tar files from Samsung and you can flash them using ODIN. Since these are official ROMs, they will not increase the knox meter.
Stock ROMs are nice when one want to do away with all the modifications and enjoy the Galaxy Tab S2 the way it was out of the box. No bugs and everything just works.
Flashing stock ROM will remove all data from your device as well. Just the data stored on micro SD card will remain untouched. So make sure you do an android backup before flashing it on your Samsung Galaxy Tab S2 tablet.