a private DNS server that only has records from your local services would at least prevent apps from reaching out as long as they aren’t smart enough to fall back to an IP address if DNS fails.

Yes, this. It’s important that your local DNS server does not even forward queries from the isolated subnet to external DNS, because these queries (and responses) can contain information. (“DNS tunneling”).

What will this mean for Lemmy instances? XMPP servers? Email servers?

What if a 15 year old runs their own personal Mastodon server? LoL this is gonna be yet another entertaining Australian government shitshow.

I think a lot of comments have missed that ntfy.sh does not use UnifiedPush, the ntfy server is a UnifiedPush provider and the ntfy app is a UnifiedPush distributor.

Regarding encryption of the push message, from https://unifiedpush.org/developers/spec/android/ :

Push message: This is an array of bytes (ByteArray) sent by the application server to the push server. The distributor sends this message to the end user application. It MUST be the raw POST data received by the push server (or the rewrite proxy if present). The message MUST be an encrypted content that follows RFC8291. Its size is between 1 and 4096 bytes (inclusive).

Need some kind of fake power-down mode baked into the OS, which locks encrypted storage and switches on an unresponsive black screen tracking mode.