I'm not sure what definition of "modern" you're using but I have a 40 year old car that has the coolant reservoir pressurized. I'm not sure how you'd have it any other way unless you had a checkvalve, the plastic bottle connects directly to the radiator via a length of rubber hose so the pressure in it is the same as in the rest of the cooling system.
The check valve is the radiator cap, it seals against 7-14PSI but not against vacuum.
I think the change came with plastics being able to withstand say 21PSI, or maybe it was safety.
Classic Chevy from the 1960's coolant recovery bottle was an option, more like an expansion tank connected to the radiator cap pressure relief port. It was for farm trucks that would overheat and people didn't want to lose glycol. See Model T erupt like a geyser when they overheat.
Otherwise, the radiator cap pressure relief port hose drained onto the ground, no recovery bottle at all until around 1972 it became standard equipment. It might also be due to cross-flow radiators (no tanks on top), vehicle emissions control causing engines to run hotter, in the 70's.
Safety issue is if glycol gets on the windshield, it just smears with the windshield wipers and you can't see much of the road if it boils over in a messy way.
Lots of Toyota's under 40 years old have coolant recovery units, but unpressurized. It seems to vary with car manufacturer's mindset.
Capacitive sensing may be superior, but millions of cars on the road have float based liquid level sensors for various fluids, they work fine.
Not according to the many Youtube videos, and most float sensors are embedded in the tank - non-servicable. A moving part in hot glycol mix with vibration and sloshing, add some dirt and oil - and it looks like all sensors are problematic. New vehicles are using capacitive probes, non-contacting with the fluid, no need for a cable-gland to run wires out of a pressurized compartment. Dodge RAM uses a reed switch inside a pressure well, with a float+magnet.
The sensors using electrodes seem to not work well either, Audi/Volkwagen expensive to change the tank. The probe resistance seems to be poorly engineered and of course the whole assembly has to be replaced, even if you only had a dirty electrode.