Guest posting still works, at least in my experience, but where you actually place the article makes all the difference. Tested a few placements last year and the pattern was pretty clear. Posts on older domains that already had traffic and their own backlink profile moved rankings noticeably. The same content on fresh blogs that looked like they were built just for guest posts basically nothing. So I stopped thinking about it as "guest posts vs other link building methods" and started caring more about the actual authority of the site where the link lives. That's what seems to matter.