As soon as President Donald Trump assumed office in January 2017, Twitter archived tweets from the Obama administration on @POTUS with a new account, @POTUS44. That account retained all of its followers, and Twitter also added all of those followers to the Trump administration's fresh @POTUS account. That won't be the case this time around.
Rob Flaherty, a digital director for Biden, wrote (on Twitter, naturally) that the company won't transfer the followers from the accounts in question. He told Bloomberg that although the Biden team pushed back on that decision, the company stood firm. A Twitter spokesperson told the publication it’s working with the transition team on a number of matters related to White House accounts.
Trump doesn't actually tweet from the POTUS account often. Instead, it typically retweets posts from his own account, which has more than 88.5 million followers. Meanwhile, Trump's personal activity on Twitter, which has caused controversy on many occasions, will be held to different standards once he leaves office.
Update (5:12 PM): Twitter has issued the follow statement about the transition.
“As we did in 2017, Twitter is actively working with the US government to support the transition of Twitter accounts across administrations, so that the incoming Biden administration will have access to institutional White House Twitter accounts.
On Inauguration Day 2021, Twitter will facilitate the transfer of institutional White House Twitter accounts including @WhiteHouse, @POTUS, @VP, @FLOTUS, @PressSec, @Cabinet, and @LaCasaBlanca to the Biden administration.
The accounts will not automatically retain their existing followers. Instead, Twitter will notify followers of these accounts to provide context that the content will be archived and allow them the choice to follow the Biden administration’s new accounts. For example, people who follow @WhiteHouse will be notified that the account has been archived as @WhiteHouse45 and given the option to follow the new @WhiteHouse account.”