I actually think that there must be people who already know which max settings for UART clock are tested and safe for different Arduino boards, and that is exactly what I am asking about.
There are no such people and no possibility of such people.
This is because the actual reliability depends on the specifics of the wiring between the two systems and the actual frequency of the two system clocks.
The wiring will affect the capacitance of the signal which will affect the rise and fall time of the signals and the actual clock frequency will change from device to device and also with temperature.
So even if you would do the experiment the results would not be transferable to another pair of devices.
Finally it is poor design to communicate at the absolute maximum speed of the hardware because it can degrade over time. Working on the absolute limit for any electronic parameter is a very bad idea.