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I am getting 404 errors while checking caching of old websites. I have research on this issue and found that Google may be collecting data for mobile index and i have check server log as well and found Google bot is visiting every day on my site but caching is not available of websites.
There are some references which i found for this issue research: Cache bug of not caching, Ajay have shared the reference: https://searchengineland.com/google-makes-some-clarifications-related-to-mobile-first-indexing-300199
Tracking of Mobile First Index Activity:
I am still in the middle of this issue and it is not get resolved.