TL;DR: They were made up by the author to provide a convenient way for the protagonist to ascend and descend sheer surfaces while leaving no trace of her route.
Based on the context (needing a certain amount to cover the distance comfortable) and the need to reach them, I think they are like pitons, wedges attached to the surface that can be used as handholds and footholds.
The Blood Mirror has a bit more description:
Each climbing crescent had a string dangling from it. Pull that string in a big circle around your crescent, and the string cut it off the wall. Each time you removed it, you lost adhesive luxin, but the crescents could be reused.
That suggests that the crescents are not actually pushed into the surface, but stuck to it. Quick Google Book searches have references to her placing her hands and feet on them, and always needing to remove the crescents back off from the surface to ensure that all traces of her travel are removed.
I do not believe we currently have a real-world version of this. Pitons, bolts, camming devices, hexes, and nuts are used somewhat similarly, but require either cracks or driving the holds into the surface, and pitons are the only ones that really provide an actual foothold or handhold rather than something to anchor a rope to. The closest parallel in terms of providing handholds and footholds are climbing holds, which are permanent bolted-in installations. Frankly, we just don't have the magic glue (luxin) that attaches so firmly and so quickly, but is removed so easily, although the various "Gecko" adhesive strategies are getting close. The next closest would be climbing cups, but they're not meant to be left in place, and only work on extremely smooth surfaces like glass.