There is a (supposed) error in /skins_fallback_v5/_404_not_found_disp.php, coming from at least b2evo5.0.4: around line 35 says <'widget' => 'coll_tag_cloud',>, but I guess the right widget to call is <'widget' => 'coll_search_form',>, which is the standard when you don't find what you are trying to see.
I don't think this qualifies as an error but rather as a matter of taste.
Anyways, we hear you and we'll include a search field by default on the 404 in an upcoming release.
Furthermore, to make it easy to configure, we'll include a container so you can set you own widgets, including search field; quickly and easily.
In the meantime, you can always customize the 404 page of you skins to make it display exactly what you want, but I guess you know that as you already looked into the code.
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I don't think this qualifies as an error but rather as a matter of taste.
Anyways, we hear you and we'll include a search field by default on the 404 in an upcoming release.
Furthermore, to make it easy to configure, we'll include a container so you can set you own widgets, including search field; quickly and easily.
In the meantime, you can always customize the 404 page of you skins to make it display exactly what you want, but I guess you know that as you already looked into the code.