I bought myself a laptop with an OLED-panel (XPS 7590), unbeknownst that brightness adjustment actually isn't supported in Linux.
This is something that I can live with, the xrandr hack is working fine, although with a caveat, which is reduced color depth at low brightness.
But I'm very curious to why this has been the case for so long, and why I can't seem to find any mailing lists or ongoing discussions around this fundamental flaw. OLED has been out for a while now, and laptops with OLED is starting to become commonplace, at least in the high-end market. It would be terribly disappointing for many Linux users to find out that their fancy new laptop is lacking something as basic and essential as screen brightness control.