This tool inserts the Unicode combining character U+0336 (COMBINING LONG STROKE OVERLAY) directly after every visible letter you type, so the line is drawn by the text renderer itself rather than by any formatting markup. That means it survives copy-paste into places that strip HTML or Markdown, bios, usernames, plain chat messages, because as far as the destination is concerned it's just ordinary Unicode text. Rendering quality depends on the font used by whatever app you paste into, so the strike can occasionally look slightly broken or offset between individual characters, especially with emoji or wide characters.
Each strike segment is drawn relative to its own character, and different fonts position combining marks slightly differently, so the connection between letters can look tighter or looser depending on where you paste it.
Yes, because it's plain Unicode rather than HTML formatting, it works in any field that accepts regular text, including bios, captions, and SMS.
Both use the same technique but a different combining mark, this one overlays a line through the middle of each character, while the underline tool places its mark below the baseline instead.