Found a staple left from a breast reduction I had years ago, when I had a mammogram. Is it safe to leave it?

Generally speaking it is ok to leave surgical clips for vessels and they are non reactive.  There is a distinct difference between surgical clips and skin staples so make sure you delineate the 2.

This is a good question and you have a retained staple most likely from the closure of the reduction which was not removed after the surgery and may have fallen into the wound.  This is not a problem and is not going to be a problem for you in the future even with mammograms.

Surgical clips are commonly used to place on bleeding vessels during your operation, and they can stay in place safely. It's good to know as it can become relevant if you ever undergo an MRI (although surgical clips are generally safe during an MRI - some exceptions exist of course). I've never known anyone to undergo further surgery to remove these clips as they are very small and almost never cause problems.

Small staples or “clips” are
used to control bleeding during many surgical operations. They are left in
place and do not cause any problems. It does not need to be removed!

Staples are sometimes used to control bleeding during surgery. The staple used during your breast reduction surgery is perfectly safe to remain in your body. There is no need to have it removed. Thank you for sharing your question. Best wishes.