Researchers working at both institutions mined publicly available Twitter data and were able to map user locations, identify what those locations were and work out what the user might be doing there – even if the user didn’t mention that explicitly within their tweet.
Worryingly, anonymous users were quickly and accurately identified from the location data within their tweets. This was made easier by historical data as, prior to 2015, Twitter included precise coordinates rather than a rough location.
I delete my tweets after 90 days with the knowledge that anyone, mainly data brokers, can make copies while they’re still visible. However, this paper identifies several reasons why it’s still worthwhile removing your historical data.