You are driving a vehicle in the built-up area. Is it prohibited in the presented situation to use the horn?

In the video you are driving along a street in a town, and near the end a black car pulls out in front of you from the right. This is an immediate danger in which you are allowed to use the horn to warn the driver. So using the horn is not prohibited in this situation. The answer is: no.

The Road Traffic Act allows a sound or light signal when it is necessary to warn of danger, and in a built-up area allows the horn only when it is necessary because of immediate danger. A car pulling out into your lane is exactly such a case. However, the horn alone does not replace braking - first you slow down, the signal is only an extra.

Legal basis: Art. 29 para. 1 and para. 2 point 2 of the Road Traffic Act (Prawo o ruchu drogowym)