Working with read replicas for Amazon RDS for PostgreSQL
The last two states are seen only when max_slot_wal_keep_size is non-negative. If restart_lsn is NULL, this field is null.
PostgreSQL 13: Don't let slots kill your primary
If max_slot_wal_keep_size is zero, which is the default value, then there’s no limit. I don’t recommend this, because it leads to failures when slots fill the disk.
PostgreSQL: Documentation: 17: 19.6. Replication
postgres-config update --config max_slot_wal_keep_size=2GB.