In Safari 12.1.1 for macOS (or perhaps since version 12.0), I've lost the ability to see which of Google's results I've already clicked on. The links corresponding to visited sites aren't colored differently from unvisited ones.
I have clicked on several links in the search results above, but as you can see, I am unable to determine which ones I've visited.
The distinction is visible in Chrome 74.0 as well as in Firefox 67.0.
What am I missing?
This is observed by others as well, judging by Google suggesting the string "google safari visited links not changing color 2019" when typing the first few words, and by the posts on discussions.apple.com.
maP1E bluE's solution works. If your "Show Develop menu in menu bar" is already checked, but your Develop menu's entries are disabled, including those under "Experimental Features", try to uncheck then check "Show Develop menu in menu bar" and/or to open a "New Private Window", from where you may be able to uncheck then check the "Swap Processes on Cross Site Navigation" entry.
The solution doesn't last. This is simply a bug in Safari 12.1.