https://openvpn.net/community-resources/reference-manual-for.../

--compat-mode version
    

This option provides a way to alter the default of OpenVPN to be more compatible with the version version specified. All of the changes this option does can also be achieved using individual configuration options.

Note: Using this option reverts defaults to no longer recommended values and should be avoided if possible.

The following table details what defaults are changed depending on the version specified.

    2.5.x or lower: --allow-compression asym is automatically added to the configuration if no other compression options are present.
    2.4.x or lower: The cipher in --cipher is appended to --data-ciphers
    2.3.x or lower: --data-cipher-fallback is automatically added with the same cipher as --cipher
    2.3.6 or lower: --tls-version-min 1.0 is added to the configuration when --tls-version-min is not explicitly set.