In description of original ticket binded with implementation of RequestDumpingHandler there is mention that it should generate some warning to the user when configured such as sensitive data may be logged (https://issues.jboss.org/browse/UNDERTOW-17).

When I add handler via CLI:

/subsystem=undertow/configuration=filter/custom-filter=request-dumper:add(class-name=io.undertow.server.handlers.RequestDumpingHandler, module=io.undertow.core)
/subsystem=undertow/server=default-server/host=default-host/filter-ref=request-dumper:add

then there is no warning generated when added nor when actual request to server is performed (I am looking into the server.log).
Also just in case - I have set logging level of the server.log to "ALL".

/subsystem=logging/periodic-rotating-file-handler=FILE:write-attribute(name=level,value=ALL)

but still nothing.