This W-level message alerts you to likely coding errors. The message produced by the new compiler looks like:
IBM1206I W BIT operators should be applied only to BIT operands.
The code generated by the new compiler for statements where it produces this message is the same as the code generated by the old compiler, although the latter issued no warning message.
As examples of where this message could arise and the likely coding errors that led to them, consider this code
dcl (x,y) fixed bin;
if x = ¬y then
...
if x ¬ y then
...
In the first IF statement, the bit prefix negation operator will be applied to the FIXED BIN variable y, and most likely that is not what was meant. Similarly, in the second IF statement, the bit infix exclusive-or operator will be applied to the FIXED BIN variables x and y, and most likely that is again not what was meant. In fact, both statements most likely contain typographical errors and were meant to test if the variables x and y were unequal.
Note also that if the bitwise operations were really intended here, it would probably be best to use the BIT built-in function (or possibly the INOT and IEOR built-in functions) to make that clear.