Deprecate loading more than one output

CHANGELOG.asciidoc

@@ -87,6 +87,7 @@ https://github.com/elastic/beats/compare/v5.4.1...master[Check the HEAD diff]
 ==== Deprecated
 
 *Affecting all Beats*
+- Loading more than one output is deprecated. {pull}4907[4907]
 
 *Filebeat*
 - Deprecate `document_type` prospector config option as _type is removed in elasticsearch 6.0. Use fields instead. {pull}4225[4225]

libbeat/publisher/publish.go

@@ -7,12 +7,13 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/nranchev/go-libGeoIP"
+
 	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/libbeat/common/op"
 	"github.com/elastic/beats/libbeat/logp"
 	"github.com/elastic/beats/libbeat/outputs"
 	"github.com/elastic/beats/libbeat/processors"
-	"github.com/nranchev/go-libGeoIP"
 
 	// load supported output plugins
 	_ "github.com/elastic/beats/libbeat/outputs/console"
@@ -223,6 +224,10 @@ func (publisher *BeatPublisher) init(
 	publisher.wsPublisher.Init()
 	publisher.wsOutput.Init()
 
+	if len(configs) > 1 {
+		logp.Warn("Support for loading more than one output is deprecated and will not be supported in version 6.0.")
+	}
+
 	if !publisher.disabled {
 		plugins, err := outputs.InitOutputs(beatName, configs, shipper.TopologyExpire)
 		if err != nil {