What are the decision criteria here?
Ideally we drop stanc2 as backfall options once rstan is up to date with the current stan and successfully running all those models in the CRAN world. Once that is the case we can be really sure that stanc3 is fully backwards compatible. From what I see that could close indeed (standalone functions and all that fun stuff).
But maybe we should anyways pull the trigger, so I am fine with listing that in the release notes…probably worth to run by the next Stan meeting?