About the Ads.txt Validator
The AppNexus ads.txt validator has been deprecated, and will no longer be accessible after (May 1st, 2019). The decision to deprecate the AppNexus validator was made after performing usage statistics analysis for the application over the past few months, and also in considering the number of full-featured, free alternatives that exist. A cursory internet search will list many of these free tools, including the following:
Some of these tools also include Bulk and API functionality, which will allow you to programmatically check lists of publishers or exchanges. Please reach out to your account manager if you have a questions or concerns about this deprecation. Thank you!