The text is crossed out using Unicode combining characters of a strikethrough. This allows it to display without any specific formatting support, but it does require that the font support those characters. The font you're using doesn't support the characters, so it displays boxes, instead.
Comments
Is there a reason why I'm seeing squares between each of the characters in your message? It's making it pretty hard to read...
I'm using Chrome on Android
The text is crossed out using Unicode combining characters of a strikethrough. This allows it to display without any specific formatting support, but it does require that the font support those characters. The font you're using doesn't support the characters, so it displays boxes, instead.
Firefox on iOS, I see strikethrough but the strikes are at varying heights.