es2 and es3 replica sets are at 70% capacity as of June 2019 (note those servers have so much capacity that means they still have 3.3TB of free capacity each). Predicted usage is to reach 80% around December 2019 - January 2020. Before they get to around 90% capacity, they will be converted into read-only hosts (in addition to es1 ones) and write service will be moved to a new es4/es5 service.
This ticket is a meta one to coordinate all steps needed for the migration.