When you delete a page (or post) from your site, you delete one or more URLs too. That old URL, when visited, will usually return a 404 not found error. But is that what you wanted? Maybe that page should be redirected somewhere. If not, and you deliberately deleted that content, serving a 410 header would actually be …Read: "How to properly delete a page from your site"
How frustrating is it to land on a 404 error page? We think it’s one of the most annoying things that users can experience on a website. That’s why you should do everything to prevent this from happening. Like properly deleting pages from your site and checking regularly for 404 error pages.
There are various tools you can use for that, like Google’s Search Console. It tells you which 404s bots encounter on your site. When you’ve found them, fix them! You can do so by fixing the link pointing to the page (if the cause was a wrong link) or, if that isn’t possible, create a redirect to the most fitting page.
Yoast SEO Premium makes fixing your 404s a piece of cake. You can get the 404 errors from Google Search Console straight in your WordPress install and redirect them right away. In addition to that, it adds an button in your WordPress toolbar when you’re on a 404 page, that allows you to instantly redirect that URL.