yes, this. if it's fallen to the sidewalk, yes, sure. If it's planted in the curb hell strip opposite a house, absolutely.

although I actually disagree about "or was very reachable from the sidewalk". Plenty of people have apple or peach trees in their front yard that overhang the property line, but if the plant is clearly in the yard, then ask.

(I have some exceptions. I urban forage from the sour cherry trees planted in hell strip of the alley beside an apartment building in what is clearly not a cared for garden; the cherries, as far as I can tell, are picked by urban foragers and robins, and nobody else. But I don't pick from the cherry tree overhanging from a fenced yard a few doors down from that apartment. I'd say that fruit planted in semi-communal spaces (apartments, parks) that don't appear actively cared for are absolutely fair game. But in a smaller property (anything from a single-family home to a tripledecker, probably) if i saw peaches falling to the ground regularly and I wanted to pick them, I'd ring the doorbell and ask.)

(note to self: next time you pass by that duplex on broadway with the peach tree, ring the doorbell and ask!)