We recognize the frustration around limitations we’ve placed on this sized unit on mobile webpages, and starting May 2, 2017 we will no longer disallow this ad unit from being placed above the fold on mobile web pages. After careful review, we've determined that when 300x250 ads are implemented above the fold in a user-friendly way, the ads do not annoy, distract, or result in ad performance issues.
https://adsense.googleblog.com/2017/05/may2017-policy-update.html