Venmo pushes hard for transaction activity to be Public. It doesn't say whether any past payments were actually public, and puts up an interstitial to confirm a change to Private.
This selection does have a benefit for the user, in making it more straightforward for friends to find each other and to make payment arrangements. However the choice has a larger impact on Venmo's user growth, and does come with downsides for their users like making activities public which they assumed were not.
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Presumably Venmo has data on how much of a network effect they get from having payment information be Public, drawing in friends and family and acquaintances and randos. Venmo appears to allow this data to impact their UI design to steer users toward the choice most beneficial to the company.