Has Epic dropped support for Windows 8.1? No engine version shows up in the launcher.
I know the spec sheet specifies either Win 7 or Win 10 but it doesn’t explicitly state that 8.1 64-bit isn’t supported.
I can confirm that Unreal Engine 5.2.1 installs and works perfectly fine on Windows 8.1. However, if you notice an unusually low frame rate within a simple starter project, just go to Edit > Project Settings and search for “Anti-Aliasing”. Change the “Anti-Aliasing Method” from “Temporal Super-Resolution (TSR)” to “Temporal Anti-Aliasing (TAA)”. My frame rate went from 50 fps to 114 fps just like that. 
PLEASE NOTE: As far as I know, Nanite will NOT work on DirectX11, so as long as your project doesn’t require Nanite, you’ll be fine. I disabled the Nanite setting in my project and set the “Default RHI” for Windows to be DirectX 11 instead of DirectX 12.
Leaving “Lumen” as the default “Dynamic Global Illumination Method” and default “Reflection Method” seemed to produce a better frame rate than “Screen Space” (at least on a simple scene).
However, changing the “Shadow Map Method” from “Virtual Shadow Maps” to “Shadow Maps” seemed to have no noticeable effect (again, on a simple scene).
Since I like the result of “Generate Mesh Distance Fields” on Ambient Occlusion, I leave that checked. But the settings I just mentioned can be looked at depending on your project and complexity to potentially help increase performance in UE5.
Lastly, if you don’t use a “Volumetric Cloud” actor in your scene and use the old BP_SkySphere instead (you can migrate it from UE4 or just enable “Engine Content” in the “Settings” option of the Content Browser and find the old BP_SkySphere in there), you will also gain some performance (in my case, from 114 fps to 130 fps). (Tip: Unreal is capped at 120 fps by default, but you can check your max by typing “t.MaxFPS 1000” (without quotes) into the console and hitting enter).
See this topic for further useful info when transitioning from UE4 to UE5: Upgrading Your Project to UE5 | Talks and demos
As a final tip, since the latest versions of some GPU drivers are not available on Windows 8.1, you can remove the constant “update your graphics driver” warning by adding r.WarnOfBadDrivers=0 to the ConsoleVariables.ini file (located at: C:\Program Files\Epic Games\UE_5.2\Engine\Config ) 