I started by clicking Tools → Customize → Toolbars, then select Text Box Formatting option and add Strikethrough.
It appears checked along with bold, italic and others.
After that I went to the toolbar, right click → Visible buttons and Strikethrough is not available.
I’m using LibreOffice 220.127.116.11 in a Windows 7 Home Premium 64 bit laptop.
Could you please help me out?