Start embedding links in the text of your old content that point to your new content (when contextually appropriate, of course). Not as warnings, but if for some reason you mention a topic or word that's relevant to a newer article...
And his design is so clean that the flags are obvious without being obnoxious.
Google does not like my new topics it seems, starting around 2012, but thankfully unlike the OP who I have a lot of sympathy for, my blog is for fun and not livelihood.
We write for bots, so they can decide to share with our fellow humans (or not), such is our lot...
Seems like something a Jekyll or Hugo plugin could do quite well.