You should be able to set "smtpd_data_restrictions = reject_unauth_pipelining" in your main.cf
This option is available in "older" postfix versions and even works with postfix 2.10.
Don't know if it is as good of a measure as the 'smtpd_forbid_unauth_pipelining' that is recommended for newer versions.
For example, Ubuntu 20.04 (LTS) is on 3.4 and 'smtpd_forbid_unauth_pipelining' is not available at all.