I'm not aware of any formal research. In my experience, it doesn't significantly affect success rates. I still strongly advise against using it, though, because it is essentially empty calories for the user (as an activity to focus on during activation).https://twitter.com/philfreo/status/1184220140868636672 …
Have some notion of trustworthiness of an account and a session, and gate the feature which you have a security concern about behind an email verification. (This assumes there is some material surface area in the app they could happily use in signed-up-but-unverified-email state)