I totally understand the appeal, but companies tend to avoid this sort of flow because it reveals information to potential hackers (or usually in this case, so called social engineers).
By not having an API endpoint that takes an email address and returns a status indicating an email address exists with Monzo or not, it helps blanket protect Monzo customers from simple phishing emails.
The API (and app) should just always say “great, we’ve sent an email”. You can test this by entering a completely made up email address - the app will not say “it doesn’t exist” - even though Monzo knows and won’t even attempt to send an email.
This behaviour is common security practice and when it is not followed, usually indicates to me the company may not be as security conscious as I would like, and potentially not follow standards elsewhere. But that’s just my own observation and is mostly anecdotal.