while this mostly affects https://github.com/matomo-org/searchengine-and-social-list/ I think this should be changed in Matomo itself
reported in https://forum.matomo.org/t/search-engines-in-the-list-of-websites/37192/12?u=lukas

Matomo doesn't detect a lot of search engines anymore as the detection assumes that the referrer URL contains the search query specified in the search engine list. But hardly any search engine sends it anymore, so it doesn't work.

As a workaround it seems like the hiddenkeyword keyword was added to the yaml file to specify the URL that is used to do the redirect over and which will be then the referrer URL.

But as referrer policy and such things are now widespread, no one opens links via redirects anymore and this is again pointless.

One could fix this issue by adding

hiddenkeyword:
  - '/^$/'
  - '/'

to every search engine in the list, but at this point the setting becomes useless and instead I think Matomo should instead handle every referrer URL like https://www.startpage.com/ as a search engine independent on the exact URL or query parameters.