Update (Jan 21, 2020): Clarified impact in more detail on both APIs.
On January 27, 2020, 
message extensions will be sunset in 
AdWords and 
Google Ads APIs and 
Google Ads scripts. After the sunset date:
  - All existing message extensions will no longer serve.
- You will not be able to create new message extensions or update existing extensions.
- You will still be able to retrieve data for existing message extensions until the end of 2020.
- You will also be able to remove the existing message extensions until the end of 2020.
What will happen when you try to create or update message extensions after the sunset date?
If you try to create or update a message extension, you will see the following errors:
Google Ads API
  - Trying to create or update a TextMessageFeedItem will result in the ExtensionSettingError.INVALID_FEED_TYPEerror.
- Trying to create a message extension via FeedMapping with placeholder_typeset to MESSAGE will result in theFeedMappingError.INVALID_PLACEHOLDER_TYPEerror.
- Trying to create or update CampaignFeeds, AdGroupFeeds, or CustomerFeeds by specifying MESSAGE in laceholderTypeswill result in theCampaignFeedError.INVALID_PLACEHOLDER_TYPE,AdGroupFeedError.INVALID_PLACEHOLDER_TYPE, andCustomerFeedError.INVALID_PLACEHOLDER_TYPEerrors, respectively.
AdWords API
  - Trying to create or update a MessageFeedItem will result in the ExtensionSettingError.INVALID_FEED_TYPEerror.
- Trying to create a message extension via FeedMapping using placeholder type ID 31 will result in the FeedMappingError.INVALID_PLACEHOLDER_TYPEerror.
- Trying to create or update CampaignFeeds, AdGroupFeeds, or CustomerFeeds by specifying ID 31 in placeholderTypeswill result in theCampaignFeedError.INVALID_PLACEHOLDER_TYPES,AdGroupFeedError.INVALID_PLACEHOLDER_TYPESandCustomerFeedError.INVALID_PLACEHOLDER_TYPESerrors, respectively.
Google Ads scripts
Trying to execute the following operations will result in the error, “Cannot operate on deprecated placeholder type” (translated to the language based on your locale):
What do you need to do?
  - Retrieve all the data from your existing message extensions that you need. All message extension data will be deleted at the end of 2020.
- Ensure that your programs don’t contain any code trying to create or update message extensions after the sunset date.
As always, if you have any questions, feel free to reach out to us on the 
Google Ads API forum.
 - Thanet Knack Praneenararat, Google Ads API Team
 - Thanet Knack Praneenararat, Google Ads API Team