Is there a 'highlight' feature in iBooks on iPads?

The feature is still there in iOS12/12.1

Place and hold you finger on the first word that you wish to highlight, lift, and select “highlight” from the pop-up menu.

You can choose between different highlight colours, underline the selection, delete a selected highlight, or copy the highlighted test to a Note.

Well, having tried this on two seperate devices, I note that the highlight function is essentially the same in iOS 12 as it was in iOS 11...

If it doesn’t work for you, I suggest that you force-close all of your running Apps from the task-switcher, then force-restart you iPad.

Are you trying this with a “PDF” or a “Book”?

The highlight function is not supported by the Apple iBooks App (iOS11) or Apple Books App (iOS12) for PDF-files. Available features are highly dependent on the file-format!

did the test, it works like lotuspilot display if it's epub ebooks and how yours look if it's pdf files

not sure if ebooks let people high light pdf files prior to ios12