I have seen ATTIC_UNKNOWN_UNENCRYPTED_REPO_ACCESS_IS_OK=yes and searched for it into the web/documentation with no luck.

grep -r ATTIC_ * into the repo reveals this environment variables:

• ATTIC_PASSPHRASE : For automated backups the passphrase can be specified using the ATTIC_PASSPHRASE environment variable
• Batch automatic answers:
  • ATTIC_CHECK_I_KNOW_WHAT_I_AM_DOING : When set, answer yes to "Warning: 'check --repair' is an experimental feature that might result in data loss."
  • ATTIC_UNKNOWN_UNENCRYPTED_REPO : When set, answer yes to 'Warning: Attempting to access a previously unknown unencrypted repository'
  • ATTIC_RELOCATED_REPO_ACCESS_IS_OK : When set, answer yes to 'Warning: The repository at location {} was previously located at {}'
• Directories:
  • ATTIC_KEYS_DIR : Default to '~/.attic/keys'. This directory contains attic keys for (???)
  • ATTIC_CACHE_DIR : Default to '~/.cache/keys'. This directory caches attic keys for (???)
  • ATTIC_OPENSSL_PREFIX : Adds ssl directory to defaults '/usr', '/usr/local', '/usr/local/opt/openssl', '/usr/local/ssl', '/usr/local/openssl', '/usr/local/attic', '/opt/local'

This is what I think they do. Please solve the documentation bug adding correct description of it into web and documentation.