It was all in place at the beginning. There were buttons under each of the articles to "Promote this post", or something like that. At some point, they hid the /promoted page and removed the buttons. I'm not sure about why, but my best guess is that the promotion buttons stopped working when they split the wallet apart from the social media functions (condenser) for security reasons, and they never got around to updating it for compatibility. The only way to promote a post now is to send SBDs to the @null account (this burns them) with the URL in the memo field, in exactly the format: "@[account]/permlink".

But, of course, there's not much point in doing that unless/until something happens to enhance the visibility of the promoted posts. TBH, I'm not sure why I keep using it. I guess just because it can be done for ¹/₁₀₀₀ of a cent, so the cost is basically 0 anyway. It does get a tiny bit of enhanced visibility from the few people who are using my browser extension or @the-gorilla's alternate feed viewer.