I base that 15% on a really cool bit of government data: OHSS's "Enforcement Lifecycle Reports." It's data on what the most current outcome of a case is, by year a person arrived. That lets you look at cohorts who arrived in similar times and see what's happened since. ohss.dhs.gov/topics/immig...

Unfortunately, the most recent data only runs through the end of 2021, so it's out of date. But if you look at what the data showed for the cohort who entered during the 2014-2016 period under Obama, by the end of 2021 about 15% had relief and many cases were still ongoing—so that number will rise.