Recently, sections of it were loosening above the oven because of the steam, and I thought, water! Water took down our plaster when the AC unit's drain plugged (a few days after we moved in) so why not use it to loosen something attached to plaster? I squirted water in 2ft. areas on the ceiling with a spray bottle, waited about a minute, and used a spackling knife to peel it off in large sections. It didn't really drip while I was waiting, it absorbed so quickly. It peeled very easily in sheets and the new surface is nearly smooth because it was previously painted with enamel. This might be the stuff you can buy in five gallon barrels at HD; it seems like little smooshy pebbles encased in drywall mud. Or someone decided they didn't care for their oatmeal breakfast and spat it at the ceiling. Whichever. The recent layer of latex paint seems super important in the process; it holds water inside the oatmeal and helps the stuff come down in sheets rather than clumps.
Since the ceiling underneath is not entirely flat, bits of the mud-like stuff (plaster?) remain in divets on the enameled sage-colored ceiling, causing a splotchy look. I'll paint over it.