It probably is still there, because it makes sense. Think of it:
The graph shows the percentage of active players over account creation date. When people log in for the first time they are "active" for one month. So the last month in the graph should always be close to 100%. As more and more people lose interest the rate goes down. The retention rate you see here for a given account creation date is not set in stone and can still decrease as more players from that date stop playing.
In other words: There will always be a spike in the last six months of the data, but there will not always be a spike around march 2019.