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It looks like Google has fixed that ongoing bug with the Google cache 404ing after being switched to mobile-first indexing. The bug has been happening for the past several months and as of last Saturday, it seems Google fixed it.
I tested it myself and for every site I tested it with, it worked without serving a 404 page not found error.
Glenn Gabe first notified me of this:
Looks like Google finally fixed the broken cache issue with mobile-first indexing. Noticed that this morning while analyzing a site. You can now check the cache and actually see it (without using the hack that was documented a few weeks ago). Awesome :) @rustybrick pic.twitter.com/nw707Bc0re
— Glenn Gabe (@glenngabe) October 20, 2018
So you no longer need to do that 404 hack to see your page in the Google cache.
Forum discussion at Twitter.
