I think the main concern that was brought up during Friday’s call was that at least some push notification services require the caller to include sensitive information in the request, which would be exposed to nodes in the subnet and then boundary nodes.
If a bad actor were able to obtain this information and impersonate a trusted application the results could be pretty disastrous.
From what I can gather, Apple (specifically) generates a key pair for you. Storing the private key in a canister is also problematic.
Perhaps if one of the third party services allowed its customers to provide their own keys, we could use Threshold ECDSA to sign requests in a secure manner.
Another concern was around idempotency and having to trade off consensus/replication if the push notification server doesn’t recognize the Idempotency-Key request header.