For a custom development project (provide an implementation to support Amazon SQS instead of ActiveMQ/Kafka for the RPC/Sink patterns to communicate with Minions), it is required to support Httpclient 4.5.x (to be more precise, 4.5.2) and Httpcore 4.4.x (to be more precise 4.4.4), as these two libraries are required by another library (a.k.a. aws-java-sdk-core) which is the core of the new integration we would like to have in OpenNMS.
Currently, OpenNMS supports Httpclient 4.3.6 and Httpcore 4.3.3.
Here is the preliminary list of OpenNMS components that rely on Httpclient (based on current develop).
The idea would be update the POMs with the new versions, and then fix what it is required to be fixed in OpenNMS.