No idea about user dropoff studies.

Slack did (do? I no longer use it) it, Scaleway (https://www.scaleway.com/en/docs/console-magic-link-authenti...), medium.com, https://www.topicbox.com/ at least.

Yes (unless you are launching an email provider or an end to end encrypted service), because it reduces the number of passwords users have to keep (one of the biggest problem of our time, password managers are not a viable / good solution) and it reduces your attack surface (you no longer have to store password hashes nor can't chose an insecure hash function).