Вот один из способов
data have;
input id $ Date : mmddyy10. Evt_Type $ Flag $ Amt1 Amt2;
format Date mmddyy10.;
infile datalines dsd missover;
datalines;
101,2/2/2019,Fee,,5,
101,2/3/2019,REF1,Y,,5
101,2/4/2019,Fee,,10,
101,2/6/2019,REF2,Y,,10
101,2/7/2019,Fee,,4,
102,2/2/2019,Fee,,25,
102,2/2/2019,REF1,N,25,
;
data want;
do _N_ = 1 by 1 until (last.id);
set have;
by id;
if flag = "Y" then _iorc_ = _N_;
end;
do _N_ = 1 to _N_;
set have;
if _N_ le _iorc_ then output;
end;
_iorc_=1e7;
run;