How to delete 404 pages permanently

Use Google Search Console to remove the URLs from your search index. https://search.google.com/search-console/about

As long as the page link returns a 404 then just wait for Google to clean up the mess on their end.

I think you can do something from webmaster tools also but there’s nothing to do on the WordPress side unless you have something comparable to redirect those to.

@sterndata, The Removals tool of Google Search Console provides only a temporary removal.

If those pages (and links to them) are no longer on your site, they’ll not be reindexed

So I will wait until Google delete them.

Thank you for your answer @sterndata.