Rules in your robots.txt file will no longer be followed by Googlebot. That’s particularly important for users who relied on robots.txt to keep search bots from discovering and/or indexing particular folders and files. The correct method will be to use indexing directives in meta tags.

For example, if you use WordPress, the main SEO plugins (like Yoast or All-in-SEO) will provide easy methods of activating metatag noindex directives for the right folders.

From Google,

In the interest of maintaining a healthy ecosystem and preparing for potential future open source releases, we’re retiring all code that handles unsupported and unpublished rules (such as noindex) on September 1, 2019. For those of you who relied on the noindex indexing directive in the robots.txt file, which controls crawling, there are a number of alternative options:

  • Noindex in robots meta tags: Supported both in the HTTP response headers and in HTML, the noindex directive is the most effective way to remove URLs from the index when crawling is allowed.
  • 404 and 410 HTTP status codes: Both status codes mean that the page does not exist, which will drop such URLs from Google’s index once they’re crawled and processed.
  • Password protection: Unless markup is used to indicate subscription or paywalled content, hiding a page behind a login will generally remove it from Google’s index.
  • Disallow in robots.txt: Search engines can only index pages that they know about, so blocking the page from being crawled usually means its content won’t be indexed.  While the search engine may also index a URL based on links from other pages, without seeing the content itself, we aim to make such pages less visible in the future.
  • Search Console Remove URL tool: The tool is a quick and easy method to remove a URL temporarily from Google’s search results.

The official Google announcement here:
https://webmasters.googleblog.com/2019/07/a-note-on-unsupported-rules-in-robotstxt.html

The Google tweet here
https://twitter.com/googlewmc/status/1145950977067016192