Voltage source corruption and change of value

Hello

Assume you have a 6,54VDC Voltage source and you short-circuited it so the output voltage after a while became 5.01

Is this voltage source "neat" to be used?
Can it be connected to a sensitive electronic circuit containing mcu for example

Certainly it is well within 10%, but how can you guarantee that? For now it will work.

But Copper wire increases resistance +4% per deg C and diodes increase voltage drop with current, as well as RC decay time is faster, so both may increase in voltage drop with current. ( or lower output voltage)

Some over voltage protection limiter can improve, but these often drop too much power or voltage unless carefully designed with a comparator + series pass high side driver or an LDO regulator with < 0.25V drop ( harder to find ( as most are >2V)) or series pass P type switch with a programmable zener may give better LDO regulation.

So as long as you maintain that load, the voltage should be as stable as the input AC voltage and output current which can vary 10% easily. BUt without this, the simplicity and reliability is the tradeoff with unregulated supplies.

Beware of overheated thin secondary windings and rectifier diodes for reliability reasons.

if you short circuit a voltage source you should get 0V, or its not a short circuit. I think what you mean is at some output current the voltage falls to 5.01V. The question is how does this current compare to the current taken by the load? With cheap power supplies the voltage falls rapidly at low currents, then falls slowly at higher currents. If your MCU does not take enough current it will get over voltage on it, which will reduce its life (perhaps to seconds!)
Frank