Noise

I'm using XT1 of the uPD78F0058 left open, but is noise influence easily received in this case?

Yes, noise influence is easily received.

EMI noise reduction measures are implemented in the uPD7800XX Series, but concretely what methods are employed?

The following countermeasures are implemented.
(1) The circuit's drive current has been revised and reduced.
(2) For ports where a large current flows, GND and power supply pins are separated.

Looking at the current of the ground pattern with a spectrum analyzer, one can observe the harmonic at 2.5MHz intervals.
Why is the current harmonic at 2.5MHz intervals even though the CPU is operated using 5MHz without frequency division?

The CPU processes multiple clocks as one unit.
For example, when NOP instructions are executed continuously, instructions are fetched in units of 2 clocks, and other than the moment of instruction fetch, almost no work is performed.
In this case, a large current (noise) flows at intervals of 2 clocks, in other words at intervals of 2.5MHz, which is half of 5MHz.

Does the internal WDT noise immunity differ between one-time PROM products and mask ROM products?

Yes, this is possible.
The difference in noise immunity may result from different processes.

I have heard that the uPD78F0034A and the uPD78P018F differ in terms of the noise resistance of their interrupt request pins. Please explain the reason for this difference.

Clocked sampling is used for the uPD78P018F's INTP0 pin, so a pulse width of at least two sampling cycles is required to ensure that interrupt requests are properly acknowledged.
This enables any noise during a period less than one sampling cycle to be completely eliminated.
However, this causes the request acknowledge timing to be slowed, and some restrictions may apply, such as not being able to use this function during STOP mode.

By contrast, in the uPD78F0034A, sampling is not clocked, so interrupt request signals can be acknowledged immediately.
Consequently, when noise that exceeds the threshold level of Schmitt trigger input is input, the noise may be received as an interrupt request. When in a noisy environment, either read P00 during interrupt servicing to check for noise, or use an external countermeasure to eliminate the noise.