RStan 2.26 is now available on CRAN! Installation from source is already possible and CRAN is in the process of building binaries for Mac and Windows that should be ready shortly.
We recommend making sure you also have the latest version of StanHeaders installed:
# in a fresh R session
install.packages(c("StanHeaders", "rstan"))
RStan is still a few versions behind the latest version of the Stan language, but this release is important because it brings RStan up to date on the new syntax for arrays that has already been recommended in the Stan documentation for many versions and is now required in Stan >= 2.33. Work on getting RStan fully up to date is already in progress.
Thank you in particular to @andrjohns, @hsbadr, and @bgoodri for making this happen, thank you to the many other contributors to this release, and thank you to RStan users for your patience!