I'll connect it to an open collector. And for the purpose of saving power, the current shall be as low as possible, as well into the base of my output, as well as through the collector from DIN1. To make sure to drop well below 1.2 V, i need the input resistance. Also, if someone has long leads between the switch and the input, it can be interesting.
I measured it myself in the meantime:
9.2 kΩ is actually not very high, so it's not that easy to pull it down, You need <7.2 kΩ or >160 µA to ground to come below 1.2 V. This info is missing in the documentation.