wrote:

Statistically speaking, I get your drift that it isn't worth removing
shards from a tire just to prevent a slow leak every couple of years, but,
anecdotally, what do you think this shard would have done, over time?
http://wetakepic.com/images/2018/02/18/splinter3.jpg

i) it would have fallen off.
ii) as the tyre wears it would have become the high spot and be ground
off by the road surface

Hmmmm.... maybe it would have fallen off, which most pebbles, I'd wager,
would do. But this would likely, IMHO, have simply been pushed further into
the tire, particularly given it's arrowhead shape.

Personally, based on the shape and direct angle of entry normal to the tire
surface, I give that shard from a 95% to 99% chance of having punctured the
tire over time, whereas I give a round pebble only from 1/10th to 1/100th
of one percent the same chance.