That feature was removed. Per Startpage support:
As of November 2018, the highlighting feature is no longer supported, as it is available in all major browsers.
Just because the browser can do it, doesn't make it convenient to do it as an additional, separate step, so maybe not a great decision on their part. But there it is.
You can try to use this userscript. It basically does exactly what you want. You'll need an extension to run custom userscripts though, like violentmonkey
Here is the content of that userscript (version 1.6):
// @name StartPage Beautifier
// @namespace https://framagit.org/SecT0uch/StartPage-Beautifier
// @description ...