The same happened to me, right around the end of February/beginning of March. A website of mine which was consistently in the top 2 results for certain queries for the past 4+ years and is the most relevant result for that query abruptly disappeared entirely. It's got modern HTML/CSS, is responsive, and uses HTTPS. It has frequent activity (it's a community) and so is constantly updated.

Looking at the indexed pages, it seems to be stuck in some sort of strange limbo where half the time the homepage isn't even indexed, then it shows up again, then it's gone, back and forth endlessly for the past few months. It never appears in most Google results anymore unless I get very specific (like searching for the name of the website word for word).

I'm at a loss for what to try next. Traffic has dropped to 1/6th of what it was before.