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Simple's logo has been designed with "responsiveness" in mind: the more space they have, the more complex their logo is.

For example on https://www.simple.com/, the logo in the header is simpler that the logo on the credit card.

They also show the 3 versions they use in a blogpost: https://www.simple.com/blog/simple-branding

They seem to be using "The Hobo’s Method" as well.

Do we know who did their branding?

According to Brand New [1], the branding was done in-house by Ian Collins [2].