This also makes it pretty clear that the deadline is May 29th until the NBA and NBPA changes their rules.
https://ncaaorg.s3.amazonaws.com/enforcement/2019ENF_NBAMemo.pdf
Pg. 13 #3 - A student-athlete who wishes to remove his name from the NBA Draft AND maintain NCAA eligibility must submit a written statement to the NBA by 11:59 p.m. Eastern time, May 29, 2019.