In Aug of 2015 I posted the spine readings of this virgin pack of Easton Axis 340s. This was done on my Flite Rite Professional scale. The scale is not in the same as Easton uses but I did not convert the results but that isn't important to this conversation.
Shaft 1 was .290, .300, and 326 grs.
Shaft 2 was .285, .290, and 327 grs.
Shaft 3 was .290, .290, and 327 grs.
Shaft 4 was .280, .290 and 324 grs.
Shaft 5 was .290, .300 and 326 grs.
Shaft 6 was .280, .290, and 326 grs.
Shaft 7 was .280, .285, and 326 grs.
Shaft 8 was .280, .290, and 327 grs.
Shaft 9 was .285, .290, and 326 grs
Shaft 10 was .290, .300, and 326 grs.
Shaft 11 was .280, .290, and 326 grs.
Shaft 12 was .290, .300, and 326 grs.
I still have 8 shafts of this set. One has been shot excessively the others not so much, 4 I have broken and are unavailable to be tested.
Shaft 1 was .288 to .289
Shaft 2 was .325 to .330, this is the one most used still intact.
Shaft 3 was .275 to.280
Shaft 4 was .285 to .290
Shaft 5 was .280 to .285
Shaft 6 was .285 to .290
Shaft 7 was .285 to .290 and is still new
Shaft 8 was .285 to .290 and is still new
Shaft 2 has definately had a measureable loss of stiffness and is the first arrow most in practice. The rest of the arrows are largely as stiff as they were 3 years ago but are also largely unused.
So yes they do lose stiffness by shooting them.