You can disable Secure Boot on a Mac and still use iMessage.
In this state, it would still be theoretically possible to attest to some kind of unique hardware ID, as the Secure Enclave is still locked down. But even if it weren't, it would be good enough to just distribute a unique key with each device. Sure you could take it off the device, but who cares? If it got banned, you'd still need to buy a new device for a new key.
…But given the sibling comment (by pxeboot) about using iMessage in a VM, I'm not sure whether any of this is actually done.
Spamming doesn’t, however, as it requires a lot of serial number lookups and creates a very identifiable trail of behavior just to get a single device working. Scaling that process up to spamming would be unprofitable and risk being caught.
In this state, it would still be theoretically possible to attest to some kind of unique hardware ID, as the Secure Enclave is still locked down. But even if it weren't, it would be good enough to just distribute a unique key with each device. Sure you could take it off the device, but who cares? If it got banned, you'd still need to buy a new device for a new key.
…But given the sibling comment (by pxeboot) about using iMessage in a VM, I'm not sure whether any of this is actually done.