https://www.cnet.com/news/amazon-alexas-brief-mode-stops-the-voice-assistant-from-saying-ok/

A new feature within the Amazon Alexa app lets you quiet down the voice-activated assistant. When you enable the "Brief Mode," Alexa will respond to your commands with a short, beeping sound rather than her customary "OK." Reddit users reported seeing the new feature this week. 

You access the Brief Mode in the Amazon Alexa app's Settings Menu under "Alexa Voice Responses." You can also ask your Alexa-enabled device to turn on the Brief Mode. Once the setting is enabled, you can ask Alexa to control devices to which she is connected and she will respond with beeps rather than "OK" to let you know that she received and completed the task.