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.
Sadly it turns out Startpage doesn't have the capability for the search query to be displayed. I sent them a support request with the same question and got this reply:
Thanks for getting in touch with us about this feature and I'm sorry not having titles in the tabs is inconvenient.
The reason we don't have different identifying terms in the search query in ...
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 ...