score:1

from t1 in Context.table1DbSet
let ok = 
    (from t2 in Context.table2DbSet 
     where t2.Idt1 = t1.Idt1 && t1.DateValid.HasValue
    ).Any()
where 
   t1.Active
   && !ok

score:11

Query syntax version of @Amy B's answer (with !Any inverted to All):

from t1 in db.Table1
where db.Table2.All(t2 => t1.cat != t2.cat || t2.julianDte >= t1.julianDte)
select new
{
    t1.appname,
    t1.julianDte,
    t1.cat
};

score:62

Try this Not Any pattern.

var query = db.table1
.Where(t1 => !db.table2
  .Any(t2 => t2.cat == t1.cat && t2.julianDte < t1.julianDte)
);

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