It is not about compliance (existing accounts are kept, and decision to prevent signups is made purely on the basis of the IP address and not e.g. the country t

It is not about compliance (existing accounts are kept, and decision to prevent signups is made purely on the basis of the IP address and not e.g. the country that issued the credit card). It is about avoiding the promised blocking of outgoing mail from Western users of FastMail to Russian users of any other mail provider.