Internal link structure and page structure are important ranking factors for search engines and visitors according to Searchmetric's study on ranking factors that was already published 2015.
What does the internal linking force look like?
In addition to external links, there is internal linking power and various methods for evaluating these. Check in Google Search Console so that you do not have an external link that goes to a page that has been removed and no longer exists because you lose link power. If these are old sites with Analytics data, I usually pick out the largest landing pages and make calls to them to find pages with potential traffic.
You can download the links in an excel sheet from the Google Search Console. Then you can see if the site looks incorrectly designed and points a lot to a page that is less important. If you drive Niels Bosmas SEO Tools for Excel you can ping all the pages so you get 202 answers (ie that the files exist) then you will see if there is link power that goes to pages that do not exist.
Does the site have incorrect internal links?
With Search Engine Optimization toolkit you can scan through the site and see that there are no incorrect redirects or broken links. WordPress also uses plugins such as "broken link checkerBut it only sees if the link no longer works. A site that has many incorrect links works worse than one that is optimized.
Incorrect links need to be redirected with 301s, which you can do easily with the redesign plugin in wordpress. In WordPress there are a number of plugins that you can use and on this page I will do some experiments which I will return to around. You can also make redirects manually in the .htaccess file.