add warning for slack setup using classic apps + RTM

prints warining on bridge startup

put it in the same place as the very legacy webhook setup instead

Author: asdfzdfj

bridge/slack/slack.go

@@ -81,17 +81,23 @@ const (
 )
 
 func New(cfg *bridge.Config) bridge.Bridger {
-	// Print a deprecation warning for legacy non-bot tokens (#527).
 	token := cfg.GetString(tokenConfig)
+	appToken := cfg.GetString(appTokenConfig)
+	if appToken != "" && !strings.HasPrefix(appToken, "xapp-") {
+		cfg.Log.Error("App token missing `xapp-` prefix")
+	}
+	// Print a deprecation warning for legacy non-bot tokens (#527).
 	if token != "" && !strings.HasPrefix(token, "xoxb") {
 		cfg.Log.Warn("Non-bot token detected. It is STRONGLY recommended to use a proper bot-token instead.")
 		cfg.Log.Warn("Legacy tokens may be deprecated by Slack at short notice. See the Matterbridge GitHub wiki for a migration guide.")
 		cfg.Log.Warn("See https://github.com/42wim/matterbridge/wiki/Slack-bot-setup")
 		return NewLegacy(cfg)
 	}
-	appToken := cfg.GetString(appTokenConfig)
-	if appToken != "" && !strings.HasPrefix(appToken, "xapp-") {
-		cfg.Log.Error("App token missing `xapp-` prefix")
+	// print warning for RTM + classic slack apps setup
+	if strings.HasPrefix(token, "xoxb-") && appToken == "" {
+		cfg.Log.Warn("Bot token with classic Slack apps setup detected.")
+		cfg.Log.Warn("It is recommended to use modern Slack apps with socket mode Events API instead.")
+		cfg.Log.Warn("See https://github.com/matterbridge-org/matterbridge/blob/master/docs/protocols/slack/account.md")
 	}
 	return newBridge(cfg)
 }