@conroj I don’t think we can reasonably make it abstract without encoding the payload as a binary string that only us can decode. Even then, dune makes it possible for users to eventually call "private API’ functions so we likely could not event prevent it this way.
We do not intend on changing the encoding often but as @pitag pointed out, this approach is bound to eventually cause unexpected bugs. We cannot guarantee that the encoding will remain stable as we might have to adapt it to fix unforeseen issues with the initial one.
The best we can do is discourage ppx authors to rely on the encoding directly and instead use our stable API which will handle the new encoding.
I’m well aware this is not perfect but I hope ppx authors will be reasonable and understand that not complying with those directives can lead to unwanted breakage as new releases come out, impacting them, their users and opam-repository maintainers.