Okular has a highlight tool and a straight line tool. However, creating a strikethrough can be tricky because one is relying on properly positioning the line over the text. PDF xchange has a bona fide strikethrough annotation whereby text can be selected (just like the current highlight tool) but a strikethrough is produced instead. I use this tool frequently as I annotate student papers and would love to see it in okular! Reproducible: Always
Just create it yourself. Settings -> Configure Okular -> Annotations -> Add -> Text Markup -> Strike Out -> Set Color -> OK