I could see limit keyword is supported in latest version v10.5.500.107 5 of db2 by default without setting MYS compatibility vector. My installation was DB2 Express-C. Is there any announcement for that? I couldn't find any online document.
After the answer of Ian I checked the value of compatibility vector after unsetting it as below also.
db2set DB2_COMPATIBILITY_VECTOR= db2stop db2start
and found it was MYS. seems like it is default set to MYS. that's why it has limit keyword support.