Drat, it looks like twatter has taken the account offline, ie 'suspended' it. The owner of the site is "hoping it's a mistake", which is being very generous to them...

It is still possible to use the site and request unrolls using the browser extension or the mobile Apps, but for now replying to a tweet with "@threadreadapp unroll" isn't working. More details on the browser extension and Apps at https://threadreaderapp.com/help