... "Hmm, why's that? It should ..."
Just asking a question.
There's tons of anecdotal evidence that's just outright wrong, or stories passed from one person to another. No harm in asking for or listening to an explanation.
This is relevant to Fords. Is it relevant to Opels?
You have lights and other electrical load for some measure of ballast. How big of a load is required to not have it trip out?
The load comment is what I was going to suggest. If you have the irresistable urge to disconnect the battery with the system running, then at least have some other ballast on the system. But the car battery is by far the best ballast as it will vary its rate of energy absorption hugely with just small increases in voltage.
The whole issue I want to point out with the voltage spiking up is simple. WIth a given load, the field current is set by the regulator to support that alternator current output at the regulated voltage. If the load current suddenly ceases, then the field current it still there for a fraction of a second until the regulator reacts, and that creates the systme voltage spike. The 'reaction time' is set by design and construction; the old electro-mechanicals just had inherent time delays in the relay action (and maybe some thermal time constants). The electronic regulators will have a low pass filter with a time constant set in them. This is on purpose because without the time delay (low pass filter) then the control system can become unstable. But it results in an alternator output voltage spike until the regulator reacts to drop the field current.
If you have other load changes, like turning the lights off, then the battery is there as a ballast to effectively absorb the spike.
As for the Ford electro-mechanical failures, I took a failed one apart, and found a resistor burned out. I did not have the experience then to figure out the exact failure but I now suspect the voltage went high and the regulator went 100% to a certain state, trying to get the field current to go way down (or perhaps even negative, if the alternator was self-exciting any), and it finally burned the part out. The electro-mechanical regulators are designed to be constantly switching back and forth between 2 states and they just vary the duty cycle to adjust the field current. Being stuck in one state is not what they are designed for.
WiIl it do it on the Opel system? IDK, as I never disconnected the battery in my Opels (or any car) while running, after learning to not do that. If you have succeeded, then it may have survived under the particular circumstances at the moment. If the battery was charged at the time, and not drawing much current, then the current change, the field current change, and the voltage spike will be small. But no one can say it will be small enough to not cause damage if the battery happens to be charging hard when it is disconnected. So IMHO no one should accept that as a generally good thing to do, and IMHO, no one should be advocating it as an troubleshooting technique.
Especially when all you have to do is grab a voltmeter and learn how to interpret the measurement results. Voltmeters are cheap and plentiful nowadays, and easy to field calibrate: check the voltage on a brand new 9V batttery; it ought to be 9.6-9.7 volts. If your voltmeter reads that, then your voltmeter is reasonably well calibrated for good field measurements..