Verizon Media (aka Oath, aka Yahoo & AOL) folks report that the merger of the AOL user base into the Yahoo Mail infrastructure is complete! Congrats! I'm sure it was a long, hard road.
The bad way to do this would have been to keep the AOL accounts alive on some old, deprecated server cluster that people will eventually forget about, and some day it would crash and data would be lost. So while it was harder up front to immediately consolidate everyone onto one single platform, I'm definitely sure that in the long term, it's for the best.