How to disable Top Sites in Safari on High Sierra

There was a way to disable TopSites, but I don’t know if it still works:

defaults write com.apple.Safari DebugSnapshotsUpdatePolicy -int 2

to prevent safari from creating the previews (and using a placeholder).

To undo the above, you need:

defaults delete com.apple.Safari DebugSnapshotsUpdatePolicy

Other than that, there is nothing else you can do.

While not a clean solution and not properly preventing Safari from creating the thumbnails in the first place, this apparently works to keep them from taking up disk space:

rm ~/Library/Safari/WebpageIcons.db && ln -s /dev/null ~/Library/Safari/WebpageIcons.db

Incidentally, even going to Safari → Reset Safari… and ticking Remove all webpage preview images will apparently leave this hack intact.

As for Top Sites, there are a few places you can remove it from:

• Safari → Preferences → General → New windows open with
• Safari → Preferences → General → New tabs open with
• Safari → Preferences → Bookmarks → Bookmarks bar

I don't believe the menu item can be removed this easily.