Add inputs.io to list of deprecated plugins.

config/config.go

@@ -1143,6 +1143,9 @@ func (c *Config) addInput(name string, table *ast.Table) error {
 
 	// Legacy support renaming io input to diskio
 	if name == "io" {
+		if err := c.printUserDeprecation("inputs", name, nil); err != nil {
+			return err
+		}
 		name = "diskio"
 	}
 

config/deprecation.go

@@ -120,6 +120,11 @@ func (c *Config) collectDeprecationInfo(category, name string, plugin interface{
 		c.incrementPluginDeprecations(category)
 	}
 
+	// Allow checking for names only.
+	if plugin == nil {
+		return info
+	}
+
 	// Check for deprecated options
 	walkPluginStruct(reflect.ValueOf(plugin), func(field reflect.StructField, value reflect.Value) {
 		// Try to report only those fields that are set

plugins/inputs/deprecations.go

@@ -7,6 +7,10 @@ var Deprecations = map[string]telegraf.DeprecationInfo{
 		Since:  "1.7",
 		Notice: "use 'inputs.jolokia2' with the 'cassandra.conf' example configuration instead",
 	},
+	"io": {
+		Since:  "1.20",
+		Notice: "use 'inputs.diskio' instead",
+	},
 	"http_listener_v2": {
 		Since:  "1.9",
 		Notice: "has been renamed to 'influxdb_listener', use 'inputs.influxdb_listener' or 'inputs.influxdb_listener_v2' instead",