We've been scratching our head with 404s that continually pop-up on Google Search Console. These are very old links (4+ years), that no longer exist and we have ensured that nothing on the current live site links to them by mistake. But they continue to appear in Google Search console.
We tried letting Google know to not index these pages by using the Google URL Removal tools, but they still appear as 404s on GSC. https://support.google.com/webmasters/answer/1663419?hl=en
For some of the links, there is a "Linked From" tab with links from our site that presumably link to the .php page reported as a 404 but we check those pages and no .php links there.