Display custom meta box

I’m really concerned about the impossibility of displaying custom meta box with the new editor. I’ve seen that adding meta boxes through the add_meta_box() function, doesn’t display them in any custom post type. And I need them to show because otherwise I should make anew all websites I made in the last two years.

In other words, how does WordPress 5 manage the meta fields? How can I display and edit them in the backend?