Simply placing an empty index.html file in a folder prevents Apache from generating an index. To make it invisible to users, use a one-pixel redirect or a "404 – Not Found" HTML page.
With the rise of cloud storage (S3 buckets, Azure Blob), the classic Apache index of files link is evolving. S3 buckets, for instance, can be configured to show XML-based listings. Tools like s3cmd and rclone have become the new wget for these indexes. index of files link
However, S3 misconfigurations have caused massive data leaks (e.g., Booz Allen Hamilton, UFC). The "index of files link" concept now extends to JSON-based bucket listings. Simply placing an empty index
Meanwhile, search engines have started de-indexing many open directories to reduce abuse. Google currently penalizes sites with directory listings unless explicitly allowed via robots.txt. S3 buckets, for instance, can be configured to
Far more common: A web developer or system administrator forgets to disable directory listing for sensitive folders (e.g., /backup, /config, /private_uploads). What they intended as a private storage area becomes a public index of files link, searchable by anyone.