How strikethrough text works
Each letter is followed by a combining character, a mark with no width of its own that the font draws over the letter before it. The classic strikethrough uses U+0336 (combining long stroke overlay), so S̶ is really two characters: an S and a stroke. Spaces are left alone, which is why the line breaks between words.
The other styles work the same way with different marks: U+0334 for a wavy line, U+0338 for a slash, U+0332 and U+0333 for single and double underlines, U+0305 for an overline and U+033D for a small x above each letter.
Ideas for crossed-out text
A struck-out word says one thing while meaning another: "I h̶a̶t̶e̶ love Mondays." Strikethrough also shows edits and corrections, old prices next to sale prices, finished items on a to-do list, and running jokes in bios and captions.
Underline and double underline can stand in for titles or highlight a key word. The overline has a math use as well: x̅ is a common way to write the mean of a set of numbers. For lines that stand on their own, such as dividers, browse line symbols.
Limits to keep in mind
Because every crossed-out letter is two characters, strikethrough text uses up character limits twice as fast. Fonts also place the marks differently, so on some devices the line can look broken, too high or too low.
Screen readers may read the words without any hint that they are crossed out, or announce each mark, which can flip the meaning of a joke. Where real formatting exists, use it: Discord strikes through text wrapped in double tildes (~~like this~~) and WhatsApp uses single tildes (~like this~). For heavier effects built from the same kind of marks, see the glitch text generator.