I’m aware that fortnite can be played cross-platform. Does the primary/official eSports format feature cross-platform play?
Yes and no.
The Arena modes, which are ranked and are required to qualify for tournaments, are fully cross-platform.
The online tournaments are separated by region and by platform, so mobile players play against mobile players, console players play against console players, PC players play against PC players, etc.
The major World Cup event in summer 2019 was played on PC. Most players used keyboard and mouse and some used controllers.