Earlier, because the site returned a large number of 500 errors to Google, Google’s indexed pages and display dropped. According to Google Search Console data it’s now fully recovered. The site was not penalized by Google (not “banned”). Thanks to the friendly webmasters for their trust; this blog firmly opposes SEO cheating.
Those days it suddenly returned many 500 errors and I never knew why, but it’s fully recovered now and the 500 errors are gone.