The smb.conf man page fails to mention that this option is deprecated even though the "testparm" tool reports this warning.
If the option is not deprecated then testparm should be fixed.
If it is then the smb.conf man page should mention that.
I believe this also carries over into Samba 4.
Fixed in 99d0c3b0dc8ffc064bd5ae31a59ed0eae56bea96 for Samba 4.4