There are many people who decide to customize their 404 error page but adding fancy pictures, animations or links to other webpages. John Mueller from Google mentioned that you can put whatever you want on a 404 error page because Google does not take the contents of 404 error pages into account. I think that most of the time people press “back” when they find a 404 error page. Thus, it is useless to spend time and money to customize 404 error pages. People replace their default 404 error page from Apache or Nginx with fancy 404 error pages. It is useless from a SEO point of view.
It is just important to check that there is a 404 HTTP code in the HTTP headers.
Quote from John Mueller on Twitter:
@piyush200885 You can put whatever you want on a 404: dancing cats, noindex, sneezing pandas, poems, role-playing games, etc; we ignore it.
— 🍌 John 🍌 (@JohnMu) December 21, 2015