How it works
After every letter, digit and punctuation mark goes Unicode's combining long stroke, a character that draws a line through the one before it: s̶t̶r̶i̶k̶e̶. Spaces are left unmarked, so the line breaks between words. It goes on top of any letter style, so struck-through bold or script works too, and Underline can be ticked as well.
How it looks
The line is drawn by the font of the app you paste into, so it can sit a little high or low, or leave small gaps between letters. Each struck letter is the letter plus a separate mark, so some apps count it as two characters against a length limit.
Questions
Do I need this in Discord?
Not in messages: Discord draws a line through ~~text~~ by itself. Use this for names, statuses and anywhere the tildes stay tildes.
Can I remove the line again?
Copy your original text, not the struck one. Pasting struck text back into the box adds a second line rather than taking one away.