Twitter reportedly decided to shut out the OSF after "thoughtful internal deliberation and close consideration of a number of factors." The foundation cited a note from Twitter regarding its decision, stating "Imagine how nerve-racking – terrifying, even – tweeting would be if it was immutable and irrevocable? No one user is more deserving of that ability than another."
The OSF has since released a statement on the matter:
What elected politicians publicly say is a matter of public record. Even when tweets are deleted, it's part of parliamentary history. These tweets were once posted and later deleted. What politicians say in public should be available to anyone. This is not about typos but it is a unique insight on how messages from elected politicians can change without notice.
It should be noted however that Twitter is not a government entity and, even here in the US, has precisely zero responsibility to allow accounts that it feels violate its TOS. In light of the shutdown, the foundation states that it will continue working ensure messages sent by elected politicians remain visible. Just not on Twitter.