At the end of September 2015 the Promotion field for Ad Groups will retire and will be replaced by an automated system. To continue with adding custom promotions you will need to start using Google Merchant Promotions. Google Adwords has done a test and compared the CTR between Adwords Promotions and Automated Promotions and have concluded that the automated feature resulted in a higher CTR.
The automated feature collects information from your existing data feed such as free shipping, sale prices etc… These attributes will than be shown alongside your product information in Google Web Search and Google Shopping. The information presented will be automated, meaning that controlling what the customer can see or can not see will not be possible. The only way to control the information is by updating your data feed with fresh details.
Ensure you keep the following attributes up to date
If you have promotions that you would like to show on Google Shopping than Google Merchant Promotions is available to you. This feature requires you to create an extra data feed to allow this information to be visible in Google Shopping and Google Web Search. Below is a great introduction video and a written article what is Google Merchant Promotions?