The percentage of times people run out of ammo vs the aesthetic cost of having one just doesn't make it worth it. You're out when your cartridge box is empty. Have you been firing your musket for almost 20 minutes of combat? You might be close to being out.
I'm familiar with the "magazine is light, magazine is heavy" gibberish that some games like to spam in text form, why? It really isn't needed. Anything but a stupid counter. How about just an animation change that pauses you briefly when reloading when you're down to five or ten rounds to do a double take at your cartridge box? That in itself might get you killed.
People go full OCD on this stuff because they're used to having it. I'm over here thinking "Man, I wish we could get that stupid compass off the screen and these line icons out of my face." This game would really, really, really suck if we started tacking all these post-it notes all over the place. Why have a feature for a concern that's a rare happening for players and is avoidable through a yet-to-exist ammunition replenishment?
It'd serve a way bigger purpose to just have some method of replenishing or sharing ammo in the first place.