Why do you have a limit on the length of a field?

So it can fit in a database, i.e. with a certain number of bytes?

If that's why you have a limit then please go and change that immediately.

No, this post is talking about having a minimum length on the password for safety reasons (i.e. a limit on the minimum entropy). You're right that a minimum byte length will ensure this, but what happens when your user types in n-1 "things" but their password gets accepted anyway. That's only a minor thing but (and I'm not entirely sure whether this is possible) what about when your user types in n "things" but the password doesn't get accepted because it's actually only n-1 bytes. Now the password won't be accepted and the user has no idea why.

I agree that these are relatively trivial things, but the point is that it's not as simple as "just use the byte length".

Some limits are technical (and in that case the hard limit is often bytes, but sometimes code units or code points, or broken if you told MySQL utf8 instead of bytes or utf8mb4), but in many cases, the limits are for aesthetic purposes: a post title or a username often is often required to be fairly short to look nice; in an ascii or latin1 world, those limits are usually expressed in terms of characters, but graphemes might be the right thing to limit in a unicode world.

"Your username must be 1-4cm when printed with 12pt Times New Roman."

I kind of like the idea of minimum length in cm as a password requirement.

What about "Your username must be no longer than 3 seconds when spoke out loud"

or, "Your username must not take more than 0.001ml of ink when printed at 12pt"

Without a limit on password length, an attacker can DOS you by forcing you to run your KDF on gigabyte-sized strings.

Giga byte sized strings?

Oh, no. That doesn't make sense. You need to limit by Giga grapheme strings.

They're only denying service to themselves if you run the KDF locally.