G's mobile-first and backlinks
 

G's mobile-first indexing is nothing new and unknown, but people still sell and share links in the sidebar and other places that are sometimes invisible in the site's mobile version.
Are people ignorant or I'm missing something?

mobile first indexing just means the user-agent is a mobile device, the link can still be in the HTML but hidden by media-queries on mobile.

Its likely google gives weight to these links

Just like how SAPE links can be hidden/invisible but still clearly help a site to rank