Google : Ads.txt is for AdSense but not necessary for search or SEO
Google's official webmaster channel on Twitter responded to a question about the ads.txt file. Someone asked if it was needed for SEO or search because they see GoogleBot trying to access it. The answer is no, you don't need it for Google search or SEO.
It is more or less necessary for Google AdSense but not for Google Search.
Here is what Google said about Twitter: "The ads.txt file appears to be related to @AdSense - it is not required for Google Search."
- Google Webmasters (@googlewmc) December 2, 2019
The AdSense folks are pushing you to advertise this file, but again, it's specific to AdSense and is an "IAB initiative that ensures your digital ad inventory is only sold by vendors (like AdSense) that you have identified as authorized."
But again, it's not necessary for SEO or search and if you don't have AdSense or other IAB related ads on your site - it's not necessary at all.
Discussion forum on Twitter.