If a publisher has a file in error (meaning they don't include all their sellers that are authorized), then that has the potential impact them. If a file is found that appears unintentionally broken or corrupted in some way, AppNexus will check with publishers rather than immediately disable buying from parties whose information is not found in the file.
Why isn't AppNexus enforcing ads.txt compliance for external bidder demand?
We are not enforcing ads.txt compliance for external bidders because we anticipate that many of our external bidder partners will be rolling out ads.txt detection and enforcement mechanisms within their own systems, and wanted to avoid duplication of logic.
For bidders who are buying external SSP demand via AppNexus and are building out their own ads.txt compliance mechanisms, we recommend they implement enforcement mechanisms in a similar fashion to AppNexus - if the SSP is listed in the file, then treat them as an authorized relationship via AppNexus.