Greetings
First the disclaimer: I am not an engineer, expert or in any way qualified to make a credible opinion as to the safety of any specific, group or category of bicycle stem or stems be they new, used or vintage of any material or method of fabrication or of any design. (I am also not a lawyer - please see "A Fish Called Wanda").
Be that as it may (I love the subjunctive), the following picture from Sheldon Brown, when he mentions "Death Stem", shows:
2 slits; one on the front and one on the back. Yours has 1.
From Brown and Rinard on testing failures, a summary:
"Most of the other failures were cracks in the quill tube, extension tube or the weld connecting these two tubes. Here's a sampling of the notes on failure modes:
Cracked in quill at bottom weld. Cracked in extension at quill. Welds polished around quill. Did not break.Cracked in quill, across weld.Cracked in quill, across weld.Quill cracked below bottom weld.Cracked along top of extension and on the compression side of quill weld.Cracked in quill, across weld.Cracked in extension at handlebar clamp. Across weld."Link:
Stem Fatigue Testing, by Damon Rinard
The weld appears as a weak point which is no surprise. These were new stems.
John Allen explains a group of failures on the horizontal bit:
Hazardous bicycle handlebar stems i.e., design flaws you didn't think of.
Multiple other manufacturers made comparable stems with the same design (single slit) - Pivo, Philippe, Cinelli for a very long time. There are all kinds of anecdotes suggesting superlight aluminum stems have a higher failure.Lots of stories of new ones failing too.
w.r.t. your actual question: stress relief holes are put in at the time of manufacture, before micro failure has started. In adding one now, you may or may not drill out all of the fatigued zone (which may not be visible to the naked eye).
One final note, I think it was Jobst Brandt who didn't know his stem had broken until he removed it to change handlebars.
I hope this helps. Ultimately make your best decision for you.
r