There seem to be at least two bugs in the broker configuration APIs and/or logic:
1. Broker throttles are incorrectly marked as sensitive configurations. This includes leader.replication.throttled.rate, follower.replication.throttled.rate, replica.alter.log.dirs.io.max.bytes.per.second. This means that their values cannot be read back by DescribeConfigs after they are set.
2. When we clear the broker throttles via incrementalAlterConfigs, DescribeConfigs continues returning the old throttles indefinitely. In other words, the clearing is not reflected in the Describe API.