When enabling the setting Do a "Search" with custom path instead of a Drupal Search when a 404 occurs, then the site responds with a HTTP code 302 and the following site (search results) responds with a HTTP response code 80. That leads to those search result overviews to be index by search engines which should be avoided.
Found an old issue #162013: Enhancement request to search404 providing ability to set meta noindex tag to "noindex, nofollow" → which was closed because some 7 years ago this module must have responded with a 404, but that's no longer the case.
Therefore I think we should get a <meta name="robots" content="noindex" /> in the header of such pages.
Fixed
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I see that this fix was merged into the 7.x-1.x-dev, but does not appear to be in my version (Version: 2.1.2). Is this a change that got lost? I tried installing the Metatag module to address this, but the noindex is not getting added b/c I'm using search404. Google is dinging me for my soft 404s and am trying to get the noindex on these pages as they suggest.