commands: Deprecate CLI flags canonifyURLs, pluralizeListTitles, pres…

…erveTaxonomyNames, uglyURLs

You can of course still set them in site config.

Fixes #4347

commands/hugo.go

@@ -509,8 +509,21 @@ func (c *commandeer) initializeFlags(cmd *cobra.Command) {
 
 }
 
+var deprecatedFlags = map[string]bool{
+	strings.ToLower("uglyURLs"):              true,
+	strings.ToLower("pluralizeListTitles"):   true,
+	strings.ToLower("preserveTaxonomyNames"): true,
+	strings.ToLower("canonifyURLs"):          true,
+}
+
 func (c *commandeer) setValueFromFlag(flags *flag.FlagSet, key string) {
 	if flags.Changed(key) {
+		if _, deprecated := deprecatedFlags[strings.ToLower(key)]; deprecated {
+			msg := fmt.Sprintf(`Set "%s = true" in your config.toml.
+If you need to set this configuration value from the command line, set it via an OS environment variable: "HUGO_%s=true hugo"`, key, strings.ToUpper(key))
+			// Remove in Hugo 0.37
+			helpers.Deprecated("hugo", "--"+key+" flag", msg, false)
+		}
 		f := flags.Lookup(key)
 		c.Set(key, f.Value.String())
 	}