IFTTT is of course entirely opt-in. Having spent an extensive amount of time working with them, they are culturally a similar company to Monzo and take privacy seriously.
Although the permissions list on the sign-in page is quite large, in practice IFTTT only currently are allowed by Monzo to access a subset of the things listed. We wanted to ask for wide up-front approval so that if we want to add more triggers and actions in future, you won’t be forced to sign in again and re-approve.
IFTTT will only access your data when there’s a need to (because you’ve set up an applet). The data they can access is also highly time-limited (48 hours of transactions, for example). They use the data in line with their privacy policy. You can go to Privacy policy - IFTTT to view an export of all the data they hold on you or delete all the data at any time.