404 errors 900+. Did an Audit to the site and found this

I recently did an Audit using Ahref to one of my client website and found that the website got a lot, a lot, a lot of 404 errors and redirect. This automatically tells me about a whole site remodelation and not a good one.

I did my best to solve some 404 but there are some pages that are completely gone and erased, but they are still showing in the report. What should I do with those?

I know google does hit ya face with penalty for having 404 error but user experience is a major problem while facing 404s.

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Edit: forgot to mention that website has 350+ redirects and nested redirects, don't really know how to face this