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Unnatural links to your site—impacts links
Google has detected a pattern of unnatural artificial, deceptive, or manipulative links pointing to pages on this site. Some links may be outside of the webmaster’s control, so for this incident we are taking targeted action on the unnatural links instead of on the site’s ranking as a whole
Are you planning to redirect all requests for the old sites to one site's front page(?)
Don't do a single redirect directing all traffic from your old site to your new home page. This will avoid 404 errors, but it's not a good user experience. It's more work, but a page-to-page redirect will help preserve your site's ranking in Google while providing a consistent and transparent experience for your users. If there won't be a 1:1 match between pages on your old site and your new site (recommended), try to make sure that every page on your old site is at least redirected to a new page with similar content.