The new update requires you to login to an Atlassian account and you will be locked out from using Sourcetree if you refuse to login.
Avoid if you don't have an Atlassian account, don't want to create one or don't want to be logged in.
Avoid if you don't have an Atlassian account, don't want to create one or don't want to be logged in.
Edit: Admittedly, I have IE8 installed on my work laptop. Not sure if IE11 has the same behaviour.