This is a sign of leafminer feeding, but leafminers do not cause serious harm to plants (despite creating prominent indications of their presence) and do not need to be treated. If you feel you must intervene for some reason, you can probably squish them easily enough by flattening the leaflet with a leafminer in it to kill the larva. No insecticide use is warranted, and no chemical will kill the leafminer without also risking harm to other species that are beneficial or otherwise harmless. Wild Black Locust trees tend to be quite vigorous (actually, fairly weedy), and routinely get heavily damaged by native insects consuming the leaves each summer, and they always grow back without issues unless something unrelated was damaging another part of the tree.

Black Locust will readily sucker (produce new stems from the root system) when the main trunk is cut down. You may be dealing with more suckers than saplings (young trees that grew from seed), although an overlap of both is possible. Because suckers have the resources of an entire tree's root system, they tend to appear vigorously and grow rapidly once a tree is cut down, even if the stump is ground out. If you want the suckers removed, a follow-up application of systemic herbicide may be needed one more times before they succumb and the roots die off so suckers stop regrowing. If you prefer to avoid herbicide and want to remove the suckers, cut them down and promptly remove any regrowth so they can't photosynthesize, and eventually that will starve the roots by using-up its stored energy.

Miri