There are many apparently unresolved problems in "modern" physics, though I can't claim to know everything others are working on so my "timeline" of where-things-are-at in physics could be quite different (and even in more than one dimension) from others.

Anyway, this was a problem I noticed with the NIST definitions of the speed of light. The speed of light is simply DEFINED to be PRECISELY 299,792,458 meters per second, IN A VACUUM (there's another rabbit hole there - light can't be detected in a vacuum), so I thought I'd dig into what the definition of a second was and it turns out to be the amount of time it takes light to travelling over a distance of X # of wavelengths of light of a specific frequency of Cesium atom radiation. Ok, so we now have a reference for a second dependent upon a distance. Ok, so let's see how we measure a meter. Turns out, it's similarly defined in terms of wavelengths of Cesium radiation.

So, if time is a function of distance and distance is a function of time, I can't measure them independently in order to measure a velocity and hence why the speed of light was simply defined to be a precise and unmeasurable/verifiable constant. If the speed of light was as fast as a crawling snail, that would still be 299,792,458 meters/second.

Also, in order to measure a distance determined by a wavelength of coherent light, I'd need a large quantity of photons to gain some confidence I've measured it closely ... which makes the NIST definition for distances something that is statistical and uncertain. (I also believe it may be impossible to construct such a setup without using multiple wavelengths of light - if all one had available was a single wavelength of coherent light to make measurements, distances greater than a single wavelength would appear to be superimposed upon each other and even fractions of a wavelength would require some additional material property)

That's practically just the entry to this particular rabbit hole, so don't get lost LOL! I can save further exploration for another "Down a rabbit post" if people are interested. Here's someone else who also appears to have found different potential problem with classical assumptions. I've come across so many things like this it's almost amazing that the whole place doesn't fall apart into a jumbled mess from moment to moment ... quite interesting stuff.