Despite Microsoft promising us they would leave classic SharePoint alone, they are making it harder and harder to get and use it. This is very annoying. The modern pages might work very well, but lists and libraries with huge amounts of columns and views just work better in classic, never mind wiki’s.
We used to be able to create classic site collections from the classic SharePoint Admin Centre, well that’s just been switched off and everything is now in the Modern Admin Centre.
So to create a classic site collection, go to the admin centre, Create Site and look for Other Options at the bottom of the screen. It may be hidden.
Then click the dropdown arrow and More Templates.
And there you go. Classic site collections are perfect for Policy Centre wikis. They are not dead Microsoft, stop going back on your word.