Twitter encouraged temporary hashflags during the 2010 World Cup. The feature linked three-letter country-code hashtags with national-flag imagery, giving tweets about the tournament a visual marker alongside their searchable text.
The 2010 tournament was held in South Africa from 11 June to 11 July. Twitter was still developing its hashtag conventions at the time: it had begun linking hashtags to search results in 2009, and Trending Topics were introduced in 2010. The World Cup provided a large, international event around which those tools could be demonstrated.
A common mix-up is the 2014 World Cup. Twitter also promoted hashflags for that tournament, and later descriptions often emphasize the 2014 relaunch or expansion. The question, however, refers to the earlier temporary deployment during the 2010 FIFA World Cup. Hashflags were not a separate sports event; they were Twitter’s event-linked hashtag treatment.