It's not a downgrade to security for any password length:
- If it's so short that the knowledge of the length makes bruteforcing noticeably faster, the password is so short that the total length taken would be very short regardless.
- In all other cases, it removes such a small fraction of time needed (on the scale of removing one age-of-the-universe from a process that would otherwise take thousands of ages-of-the-universe) that it doesn't change any infeasible timescale to a feasible one.
So either the information isn't needed, or it won't help. So not a security decrease.
- If it's so short that the knowledge of the length makes bruteforcing noticeably faster, the password is so short that the total length taken would be very short regardless.
- In all other cases, it removes such a small fraction of time needed (on the scale of removing one age-of-the-universe from a process that would otherwise take thousands of ages-of-the-universe) that it doesn't change any infeasible timescale to a feasible one.
So either the information isn't needed, or it won't help. So not a security decrease.