brms 2.21.0

New Features

• Add experimental support for the pathfinder and laplace algorithms
  in the cmdstanr backend. (#1591)
• Automatically recompute fit criteria previously stored in the model
  if potentially results-changing arguments are provided to the criterion method.
• Allow to turn off automatic broadcasting of constant priors.
• Allow for joint likelihood evaluation in kfold via argument joint.
• Use several Stan built-in functions implemented since version 2.26
  to improve the efficiency of multiple model classes. (#1077)

Other Changes

• Change make_stancode and make_standata to be aliases of stancode and
  standata, respectively. Change get_prior to be an alias of a new generic
  method default_prior. This enable other packages to define new stancode,
  standata and default_prior methods to generate Stan code and data, and extract
  the default priors, for their own objects building on brms. Thanks to Ven Popov
  for helping with this. (#1604)
• Change the default prior of the shape parameter of negbinomial models
  to inv_gamma(0.4, 0.3) thanks to Aki Vehtari. (#1614)
• No longer automatically canonicalize the Stan code if cmdstanr is used
  as backend. (#1544)
• Export read_csv_as_stanfit thanks to Ven Popov. (#1619)
• Make installation of shinystan optional. This means that the package has to
  be loaded, via library(shinystan), before launch_shinystan can be used. (#1595)
• Improve parameter class names in the summary output.
• Show histograms rather than densities in the plot method by default.
• Deprecate argument N in the plot method in favor of argument nvariables.
• Remove deprecated argument exact_loo in method kfold.

Bug Fixes

• Remove some remaining uses of Stan's old array syntax.
• Fix a bug in formula parsing of missing values terms with interactions
  thank to Guido Biele. (#1608)
• Ensure compatibility of combine_models with moment matching. (#1603)
• Ensure compatibility with the latest splines2 package version. (#1580)
• Fix output of rmulti_normal thanks to Ven Popov. (#1588)
• Prevent memory leaks when executing kfold or reloo in parallel.