The 2.22 release changed the default stanc compiler.
I added logic to the Makefile that would let users compile using the old stanc2 compiler, but I think that the way I handled this for the 2.22.1 release was quite clunky - it forces the user to keep recompiling
bin/stanc in order to toggle between the old and new compilers.
I implemented a better solution which went in with PR https://github.com/stan-dev/cmdstan/pull/820. This solution allows the user to pass in make option
STANC2=TRUE when compiling the model; this will build
bin/stanc2 as needed and use it to compile the Stan program.
I think it would be good to put out another minor release of CmdStan now so that this is exposed to CmdStan users, because we are seeing bugs reported for the new compiler - e.g. https://github.com/stan-dev/stan/issues/2890 (note this should be a stanc3 issue). This would make it easier for CmdStan and CmdStanR and CmdStanPy users to work around bugs in Stanc3.