At this point even though we have a new modern page, Site contributors can still create pages based on classic model like Web Part page and wiki pages. Let’s see how can we disable the same.
Go to Library Settings of Site Pages
Click on “Change new button order and default content type”
Deselect the check boxes next to Wiki Page and Web Part Page and click OK.
And that’s it. Now if any site contributor goes to create a new page in the Site Pages library, he/she will not see the options to create web part page or wiki page.
And that’s it. Your entire site now is the modern site and completely responsive 🙂