What is "WARNING 'syslog' option is deprecated"?

The syntax you are looking for is: logging = syslog@1 /var/log/samba/log.%m

Just comment out or remove syslog = 0. The number is the severity mapping for which logs the errors go into. 0=> log_err, 1=> log_warning, 2=> log_notice.

The number parameter sets a threshold for sending messages to syslog. Only messages with debug level less than this value will be sent to syslog.

Default is 1. If you are setting up a development sandbox I would just whack that stuff unless you are having problems and need to see the logs.

I'm using Debian and the above syntax shows no errors running testparm.

I had the same warning. It went away when I commented out the line

syslog = 0

in the file smb.conf.

Newer versions of Samba have a more powerful parameter named logging which provides finer control over logging. The warning tells you that in future versions the parameter syslog may be considered for removal in favor of the newer logging. See man smb.conf for details.