https://code.briarproject.org/briar/briar/-/wikis/FAQ#will-there-be-an-ios-version-of-briar

We’re looking into whether an iOS version is feasible.

A typical iOS messaging app would use a push notification to wake the app when a message is received, but this exposes metadata to Apple’s push notification service and the app developer’s push gateway

If we don’t use push notifications then the best Apple allows us to do is wake up every 15 minutes and check for messages. But maybe the sender won’t be online when we check (their 15 minute intervals might not be aligned with ours - clocks aren’t perfect).