I know someone who interviewed at Monzo and said the interview process and technical test (write a web crawler in Go) was generally a pretty good process. However they "noped!" out of there when the interviewer said that they don't do unit/automated testing, and it's the developer's responsibility to make sure their code works.

This was a while ago, so hopefully things have changed or hopefully the interviewer was not telling the truth (perhaps looking for a reaction?). However if it is the case then they're about as "modern" a bank as all the rest of these legacy banks, or at least will be in a few years time when their technical debt starts to catch up with them.