The service_plan utility is used for managing hosting plans through the command line.
When you created a hosting plan specifying
'-shell false' option to prohibit access to server shell with FTP user account credentials the shell access is being enabled and shell is being set to
# /usr/local/psa/bin/service_plan --help | grep shell -shell <shell_name|false> Allows/disallows access to server shell with FTP user account
Such behavior is considered to be the software issue, its ID is PPPM-712.
The issue should be fixed in the future Plesk Panel releases.
Please specify the shell type manually using the command below to prohibit access:
# /usr/local/psa/bin/service_plan --create <plan_name> -shell /bin/false