That hasn’t been true in the U. S. for a good 50 years. Cars going back to the 60s had a small float switch that flipped a dashboard light if the fluid got low. As one who used to work on cars as a profession, I don’t recall that I’ve ever seen a car without this simple warning device.

The reason parent commenter doesn’t know this is because hydraulic leaks on auto brake systems are relatively rare as long as the vehicle darkens the door of a shop occasionally, even if rarely. Ergo, one might not even know there is a dash light.

The lamp test, where all the lamps are turned on as the car starts, should be a sign that there is a lamp for this.

Or else how else would you know if the "brake" lamp still worked?

Tell me with a straight face that you know more than one person beside yourself that actually looks at those lights at startup and checks that they are working. :-) I mean, you're right, but I'd venture to guess that for a lot of people the "lamp test" doesn't mean anything to them.

I tend to think that this is one of those things that amateurs would only notice at first as, "I think that one of those red lights is not turning on at ignition." Not specifically the brake fluid level.

Every car I've worked on over the last 3 decades has a brake fluid level sensor (usually just a go/no-go float switch).

https://www.youtube.com/watch?v=jsGZJFMgUpA has a quick view to it on a random car in the first 11 seconds of the video.

(Something very much like) This is required by 49 CFR §571.135.S5.5

https://www.law.cornell.edu/cfr/text/49/571.135

yeah what.... it's literally just a float switch wired to the dash light

My Honda Civic had one 30 years ago.. a quick search for that part shows that it interchanges back to a 1980 Civic too.

Visually? Really? Didn't know that. I only own and work on old vehicles, and those all have a float with a magnet that triggers a reed switch.

Every car back into at least the 1980s has had an fluid level indicator switch and a light on the dash.

Grew up driving pre-1980's cars and didn't know about this until this year. The light on the dash that lit up was the same indicator as the light that comes on when the hand-brake is engaged. When the light would not go off when the hand-brake was disengaged and I thought the brake needed adjustment. Nope, fluid just low.