They should be able to slot into the headlight socket of a car from 1990 with absolutely no issue and it should be hard to tell it’s any different.

this is the only part I don't think is completely true. older headlight housings and lenses/reflectors are designed for a single high intensity point of light (well not quite a point but a tiny coil like a grain of rice size max,) and LEDs don't behave like that (they tend to require more surface area to output that much light, and are directional), so putting any ol' LED drop-in bulb in old housings is somewhat risky and usually results in unintended beam patterns that don't match the OEM bulbs and can blind other drivers. With modern LED technology it may be possible to engineer a custom drop-in that works for the vast majority of old reflector designs but that isn't what any of the ones I've seen do, they just more or less mimic the size and shape of a bulb and cram as many surface mount LEDs on as they can and call it a day.

Also this is a dumb nitpick but I don't believe LEDs (at least consumer grade ones) can offer the full spectrum distribution of sunlight or even incandescent bulbs, since LEDs each output one specific wavelength. With the right combination of LED types and phosphor coatings I think they get close? but still have more peaks and valleys than incandescent