After some discussion & analysis, we've decided to remove the "Demote Sitelinks" setting in Search Console.
Sitelinks are the links shown below search results on some queries, to help users navigate your website. Over the years, our algorithms have gotten much better at finding, creating, and showing relevant sitelinks, and so we feel it's time to simplify things.
We only show sitelinks for results when we think they'll be useful to the user. If the structure of your site doesn't allow our algorithms to find good sitelinks, or we don't think that the sitelinks for your site are relevant for the user's query, we won't show them. This process is completely automated. Sitelinks have evolved into being based on traditional web ranking, so the way to influence them is the same as other web pages.
Best practices to improve the quality of your sitelinks include:
# Provide a clear structure for your website, using relevant internal links and anchor text that's informative, compact, and avoids repetition.
# Allow Google to crawl and index important pages within your site. Use Fetch and Render  to check that they can be rendered properly.
# If you need to remove a page from search completely, use a "noindex" robots meta tag  on that page.
Should you run across undesired sitelinks for your site with common queries, feel free to drop by the webmaster help forums  to get advice from other webmasters, and to let us know about it. We appreciate all feedback on our search results, and use it to help further improve our algorithms over time!
 Fetch & render: https://support.google.com/webmasters/answer/6066468
 Noindex: https://support.google.com/webmasters/answer/93710
 Webmaster forum: https://support.google.com/webmasters/go/community