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A general description is here.
Serialization format version 3 becomes the default for all serialization and saving of the workspace. Serialized data in format 3 cannot be read by versions of R prior to version 3.5.0. Serialization format version 2 is still supported and can be selected by ‘version = 2’ in ‘save()’, ‘serialize()’ and ‘saveRDS()’. The default can also be changed back for the whole R session by setting environment variables ‘R_DEFAULT_SAVE_VERSION’ and ‘R_DEFAULT_SERIALIZE_VERSION’ to ‘2’.