Google's John Mueller posted a comment on Reddit describing what is often referred to as the "Japanese keyword hack," he said. In short, it is an automated process that looks for vulnerabilities in your CMS to inject content on your website.
John said, "It looks like you (or your hoster) already cleaned some of this up - it's often called the "Japanese keyword hack"."
What happened was this site noticed over "20,000 pages in Japanese or Chinese have been indexed" as reported on this person's Google Search Console property. He didn't built that content, someone else injected it on his site and he didn't know about it.
Here is a partial screenshot of the site command for that infected site:
The site was hacked and now this site owner needs to get the hack under control and remove the hacked content from the site.
Google even has a document on this hack saying, "The Japanese keywords hack typically creates new pages with autogenerated Japanese text on your site in randomly generated directory names (for instance, http://example.com/ltjmnjp/341.html). These pages are monetized using affiliate links to stores selling fake brand merchandise and then shown in Google Search."
Here's an example of what one of these pages looks like according to Google:
John Mueller from Google explained that "since someone hacked your site, even if you've cleaned up the hacked traces, it's important to understand how they did it, so that you can make sure that the old vulnerabilities are locked...
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