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Last tested: Oct 28, 2019
Sending our embed URLs via Slack causes Slack to ping the URL in an attempt to display a preview. When that fails, the recipient will attempt to hit the embed URL and it won't work because embed URLs are one-time-use, and Slack--in an attempt to preview the content--used that single-use URL.
Send it in a code block using the normal
code block method in Slack to preserve the URLs.