This sounds a bit complicated (of course you can just do steps 1 and 2…) but this is the most consistent way to format text.
From the menu, choose New Style form Selectionįrom now on, to strike any word or words just select them and double click on the Strike word showing in the list.
Click on the 7th icon (last) in the same row as the “a” icon (New Style from Selection).
Click on the small “a” icon (Character Styles) just under the Sidebar title "Styles and Formatting).
Click on the Styles and Formatting button (2nd colour icon from the top on the right edge of the LibreOffice window).
Select some words you want to strike, and click on the Strikethrough button (4th under the Font name).
Display the sidebar (menu View >Sidebar).
The best way to apply the Strike effect is by creating a Character Style.