Please Note: I'm not asking what to do here. I'm asking why this is happening, in order to understand Google Search Console's behaviour.
So this has been driving me nuts for weeks. I've made repeated removal requests in Search Console for 404 error pages that Google somehow sees as coming from pages long since gone, including a Yoast WP SEO XML sitemap (I even removed the Yoast plugin months ago). I've been repeatedly requesting removal of all these pages for months now, but they keep popping up as sources for my 404's.
I've run Screaming Frog, Deepcrawl, combed though the site and Google's indexes, and not seeing a trace of these pages anywhere but from inside Search Console...