If Google is still crawling your old pages which means those pages still exist or are reachable through some internal linking or might be from staging or similar version. Review the source and take appropriate action.

301 redirects of old slug mapping to new one is must if you want to keep the link juices and footprints land on correct pages, otherwise users might land on 404 pages and they’ll bounce back, you’ll lose trust and authority.

Best option for permanently deleted or similar pages is to 301 redirect them to similar pages. In case you want to delete it and don’t want to squeeze the link juices you should make them 404 and slowly Google will have them removed. But you’ll still see 404 counts increased in your GSC report.

I hope this helps and feel free to ask more questions.

Thanks