G-1050 should not apply to check constraints.

Check constraints only accept literals, sql-functions (not PL/SQL) and references to columns in same row.
see section "Restrictions on Check Constraints"

create table t (
    c char(1) default on null 'Y' not null,
    constraint t_c_chk check (c in ('Y', 'N'))
);

results in

Warning (3,37): G-1050: Avoid using literals in your code