Reverse engineering for the purpose of interoperability is explicitly allowed.
Also, breaking the TOS is usually not illegal. TOS of a random company is not the law, otherwise you would get into trouble non-stop from random websites and apps making you "agree" to things.
> Reverse engineering for the purpose of interoperability is explicitly allowed.
In Apple's iMessage TOS? I don't find that likely but open to being wrong.
> Also, breaking the TOS is usually not illegal
In general, contracts are legally binding, therefore breaking them is illegal. Sometimes contracts include clauses that can't be legally binding, but I don't think a TOS forbidding this type of behavior would be questionable in the slightest. Apple obviously has no obligation to allow anyone to use its platform as a backend for their own (previously commercialized) product.
> In Apple's iMessage TOS? I don't find that likely but open to being wrong.
In EU law. No contract or license may restrict your right to reverse engineer or decompile for the purpose of interoperability or building an alternative implementation.
Also, breaking the TOS is usually not illegal. TOS of a random company is not the law, otherwise you would get into trouble non-stop from random websites and apps making you "agree" to things.