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This is a bit of text trickery. The crossed-out circle is a special character that covers the previous letter or emoji. If you copy and paste it, the circle covers any previous character, not just the rainbow flag emoji.
Jeremy Burge, who lists his title on Twitter as “chief emoji officer” for Emojipedia, set the record straight, saying, “it's literally how that character works in combo with any emoji or character.”
Burge pointed out that in situations like this, it can be hard to differentiate which Twitter users have been genuinely fooled and which ones are pretending to be fooled as a joke. See for yourself:
The special character is part of Unicode, which is a standard set of characters computers use to handle text. The latest version contains more than 137,000 characters to handle languages from English to Arabic as well as emoji and symbols.