Is the maximum size of the SD card for the S9+ really limited to 400 GB?
I can now confirm that the S9+ supports a 1 TB μSD card just fine.
After a colleague had outfitted his phone with a 512 GB card, I decided to give it a try, because it was already above the limit Samsung claimed was supported but well below the limit one should expect for SDXC (see my question).

Edit (February 2021): Due to the other answer which appeared here, I wanted to update the screenshot and also wanted to state which model I use: Sandisk Extreme (not Extreme Pro, though).
Also note: I have meanwhile read in several places that phone manufacturers indeed specify the &quot;supported card size&quot; based on the SD cards available at time of the release of a phone model.