Guest posts still work like a charm for us, and honestly bring some of the best results out of everything we run. One client came to us with zero authority and zero traffic, and with the right content placements (relevant, well-written, on genuinely authoritative sites), we moved both their authority and traffic significantly using guest posts and niche edits alone.
Also, the site's technicals need to be solid too, since even the best placements won't fully convert if there's friction on the site itself.