According to the libpq documentation, "prefer" is the default in the
underlying library and so setting a different default in the Terraform
layer would be a breaking change for existing users of this provider
whose servers do not have TLS correctly configured.
The docs now link to the libpq manual's discussion of the security
implications of each of the ssl_mode options, so the user can understand
the limitations of the "prefer" default and can make an informed decision
about which setting is appropriate for their situation.
Martin Atkins