We started adding an `i` tag like this just last week, though it only applies to zaps coming from the mobile app. Have chatted a bit with  nostr:npub1v5ufyh4lkeslgxxcclg8f0hzazhaw7rsrhvfquxzm2fk64c72hps45n0v5 about the format. Here's the mobile code: https://github.com/wavlake/mobile/blob/main/hooks/useZap.ts#L39

There is also the `a` tag that references the track/episode id, that'll likely change though once we update how we're publishing tracks as nostr events.