Appsflyer is a marketing analytics platform according to their own website:
AppsFlyer is the world’s leading mobile attribution & marketing analytics platform, helping app marketers around the world make better decisions.
AppsFlyer’s People-Based attribution breaks down data silos providing marketers with unified access to data on every touchpoint along the user journey.
It has been added in Spybot Anti-Beacon 3.4 because we encountered apps that do not inform the user about the use of AppsFlyer. Since any contacts to third parties can be regarded as sharing PII (Personally Identifiable Information) in the European Union (IP addresses are PII), software using third party web servers can be regarded as tracking, and without proper information, consent and control, as Spyware.