Download link for older Safari Technology Preview version 51 As was answered in this question, Cannot paste images from clipboard to Facebook in Safari unlike Google Chrome, Safari Technology Preview 51 has the attribute to copy and paste Facebook and Tumblr images. Apple has recently upgraded Safari Technology Preview to release 52 and it has more Facebook related bugs than release 51. It has also lost that attribute.

Where can I download Safari Technology Preview version 51?
Use the Wayback Machine:
https://web.archive.org/web/*/https://developer.apple.com/safari/download/

Once you find the download page of the version you'd like to download, strip Wayback Machine's URL prefix, and download it directly from Apple's servers.