The “The keyword is not recognized; keyword is ignored.” is because SEU syntax checking is frozen at the 6.1 level for RPG. IBM is no longer providing updates. The HANDLER keyword was added to RPG after the support for SEU was withdraw. This means that SEU does not recognize the HANDLER keyword as valid, but it is valid and the compiler is very happy about it. IBM wants you to stop using SEU and switch to it's (expensive) code editor - Rational Developer for i.
You can ignore the SEU syntax warning when entering this new keyword, save the source and compile as normal.