wpDiscuz asking at dashboard to create a custom comments form for an Elementor templates (and maybe other Posts types too) - what is obvious wrong. Because you can't know all possible WP posts Types - seems that it better to remove this warning at all.
Could you please navigate to Dashboard > wpDiscuz > Forms admin page and check comment forms? If you don't have any comment form please create one or disable wpDiscuz. wpDiscuz requires you to have at least one comment form.
Full list of an active plugins will be too huge. I can try to disable some by your logic suggest.
Answers:
a) I see this message only at Elementor Templates and ElementorPRO ThemesTemplates lists pages. I DONT see this message at similar pages of a WP Timeline, FAQ Revo, etc plugins posts lists which also use Posts table for store their garbage.
b) No. But I want to use WPML later.
c) According to (b) at both my forms only a main WP site language are displayed: ru_RU.
d) At wpDiscuz comments forms I see only standard Posts types as: " post page attachment product", nothing like "Elementor templates" etc.