Several processor manufacturers have processors with laser-trimmed RC oscillators managing 0.5% frequency tolerance. This can be used with UART. But note that a RC oscillator may be sensitive to the ambient temperature so in automotive applications you may get too large baudrate errors. Some processors also allows a correction coefficient to be stored in EEPROM or in flash or programmable "fuse" so the factory can measure the actual frequency of the RC oscillator. The chip may then use a programmable resistor to adjust the RC time constant and gain even better results. Some chips allows this to be done during runtime too - for example measuring bit rates from the other side and try to adjust the own oscillator to reduce the baudrate error.

But as already noted - you must pick a specific chip from a specific manufacturer and evaluate. We can't give answers for wildcard processors.