A finance blogger posted in r/Blogging this year with one line: from about 800,000 readers a month to roughly 200 visits a day.
No warning. No manual action. The drop landed right after a core update.
The reply that got attention wasn't about Google at all. It said a fall that steep usually means something broke on the site, not that an algorithm turned against it.
Short answer: work out whether you lost rankings or just lost clicks, since they need different fixes. Then build channels an update can't touch, starting with the posts you already have.
I write about this for RankControl, and the plan below is the one I'd actually run, roughly in the order I'd run it.