A soft 404 is a URL that returns a page telling the user that the page does not exist and also a 200 level (success) code.
In some cases, it might be a page with little or no content--for example, a sparsely populated or empty page.
If your page is no longer available, and has no clear replacement, it should return a 404 not found or 410 (gone) response code. Either code clearly tells both browsers and search engines that the page doesn’t exist. You can also display a custom 404 page to the user, if appropriate: for example, a page containing list of your most popular pages, or a link to your home page.
By submitting a sitemap of your 404 pages to Google you can expedite the removal process.
Hope this helps,