Not an ultrawide or multi-monitor user (single 4K 27” miniLED for me), but hdr support is so close to being perfect but not quite there yet. The support has finally been added to Wayland git and is coming in the next update iirc, but at the moment it relies on your window manager’s implementation (KDE’s works great) and doesn’t work for gaming without running gamescope (steam’s window manager) in a window. The only issue I think will remain with HDR after the next update is with apps that stubbornly use X instead of Wayland (steam is the one that kills me here), since X won’t ever support it so those apps will be SDR.

In terms of OLED support, they don’t need to be treated specially to work so any of them should work as normal - only thing to be aware of is that WOLED panels made by LG (used in asus monitors too) use an uncommon subpixel layout and you may have to set it manually or fiddle with your text rendering settings a little to see it perfectly. Samsung panels (like the ones Alienware uses) use the normal layout so no concerns if you go with that. Otherwise, screen dimming / turning off after a period of inactivity is a common feature and should be good enough for protecting from burn in. The only other OS-level feature I’ve seen related to OLEDs is shifting sustained bright pixels around to share the load - not sure if anyone’s made this on Linux, it sounds awful to use so I’ve never looked into it.

Someone else already mentioned old games not supporting ultrawide well, but worth adding if you go OLED you can just run it 16:9 and the letterboxing won’t be nearly as obnoxious as on a standard IPS/VA/TN/whatever monitor that would be blasting ugly blue/black light from the “disabled” areas.