The quick version:
Google appears to be slowly dropping pages from the index entirely across many sites.
Google will not explain why, which makes it hard to diagnose.
Deindexing is far more serious than a ranking drop, because the page cannot rank at all.
Check, do not guess
Use the URL inspection tool to see whether your important pages are still indexed. If money pages or cornerstone content have dropped out, that is your priority.
Rule out the obvious first
Look for accidental noindex tags, blocked resources, or thin content Google may have decided is not worth keeping. Sometimes the cause is on your side even when a wider pattern is happening.
If your pages are clean
Then you are in the frustrating zone where the cause sits with Google and there is no public answer yet. Site owners are comparing notes on Reddit and asking about it on Quora right now, which is worth watching, but treat the speculation carefully. The reliable response is to keep your strongest pages too useful and too well linked to be quietly discarded.
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