The number of U.S. citizens who have renounced has plateaued starting in 2016. That was the peak year with 5,409 renunciations followed by a similar number of 5,132 in 2017. See an older 2014 post that highlighted the then record of 2,999 for the entire year.
The data used for these running compilations, with the individuals names published can be reviewed on the federal government’s website. The complete set of lists going back to the mid-1990s can be reviewed here. Quarterly Publications. Quarterly Publication of Individuals, Who Have Chosen to Expatriate
The total number of renunciations for the first two quarters of 2018 was 2,185.
None of this answers why these numbers have stopped increasing in a ski slope fashion?