Due to this change below in R, the rds formats may not be compatible with older R versions anymore. Therefore, the data should be saved in the older format version 2.
CHANGES IN R 3.6.0:
SIGNIFICANT USER-VISIBLE CHANGES:
Serialization format version 3 becomes the default for
serialization and saving of the workspace (save(), serialize(),
saveRDS(), compiler::cmpfile()). 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 the save/serialization functions. The
default can be changed back for the whole R session by setting
environment variables R_DEFAULT_SAVE_VERSION and
R_DEFAULT_SERIALIZE_VERSION to 2. For maximal
back-compatibility, files vignette.rds and partial.rdb generated
by R CMD build are in serialization format version 2, and resave
by default produces files in serialization format version 2
(unless the original is already in format version 3).
Due to this change below in R, the rds formats may not be compatible with older R versions anymore. Therefore, the data should be saved in the older format version 2.