Would you guys like to see a Windows 8.1 Extended Kernel? I am well versed with Windows and basic driver functionality, but not with advanced Windows knowledge needed to whip up an Extended Kernel such as incorporating Windows 10 functions. Since software support for Windows 8.1 is still great with the software that doesn't work only needing mods to either remove the version check or simple DLL stubs to run, we can make our first priority drivers, most specifically graphics and Wi-Fi drivers. All the major hardware vendors (Intel, Nvidia, and AMD) have ended support. Intel ended theirs first starting with Kaby Lake (though unofficial mods for drivers up to 10th Gen(?) exist. see this thread) while AMD and Nvidia just ended theirs at roughly the same time last year (In fact, this is being posted from a 2019 AMD laptop with 8.1 on it).
Now, it should be noted that there are some hardware manufacturers that still have support for OSes such as 8.1, and even Windows 2000 in specific instances, to this day! Notable ones include Realtek (certain hardware) and most major touchpad manufacturers such as Elantech (provided you aren't restricted exclusively to HID-Compliant Touchpad with no way to enable PS/2 touchpad).
Realtek still has backwards supports for OSes older than Windows 10, but it depends on the hardware. For instance, the Realtek RTL8892CE WiFi card (not to be confused with the RTL8891CE) is OEM exclusive and has no 8.1 or 7 support. Plus, there are some Realtek Audio drivers that integrate with the graphics card. You'll know you have one of these if there is a red circle on the audio icon right after installation finishes (and hovering over it says "No Audio Device is installed"). Because of this, the Windows default audio driver does not support this configuration and thus, will not load. The standard Realtek HD Audio driver will not work either. Realtek Ethernet still supports Windows 8.1, though it appears Ethernet may be going the way of the dinosaur due to a lack of Ethernet ports on most new laptops.
Most major touchpad vendors should still work. Grab the driver from either the manufacturers website or your laptop manufacturers website. If you have the option to switch between Advanced (HID Compliant, I2C) and Basic (PS/2) emulation touchpad, you can set it to Basic and be fine, but if you only have Advanced, you'll need a USB Mouse.
Despite Intel dropping official support with Kaby Lake, the latest chipset drivers surprisingly still work.
To install Windows 8.1 on modern laptops, just burn the ISO to Rufus with UEFI support and boot it with a USB Mouse connected (plus keyboard if installing from the Desktop). Since 8.1 has generic USB3 drivers in its WinPE unlike 7, no PS/2 peripherals such as adapters are needed. After the installation restarts, you'll get a quick flash of a BSoD. Load back up the install media, hit Shift+F10 to load up Command Prompt, type Notepad.exe, click on File/Open, select All Types instead of .txt, point to C:\Windows\System32\drivers and delete Intelpep.sys. Exit out and finish Windows install normally. If you update, that file will come back so you'll need to repeat it once more. It would be great if we could find a fix for that file that allows it to function as intended.
I hope I'm not the only one interested with Windows 8.1. Even with UEFI7, it is nearly impossible to install Windows 7 on modern UEFI boards. It doesn't help Legacy Support is being phased out so the amount of new computers with that is dwindling, meaning UEFI support has to be incorporated somehow into 7 (If you guys want to tackle this problem, please start a different thread about this. You have my backing). Plus, there are now 0x0A5 NON-ACPI-COMPLIANT BIOS BSoD's now as well.
tl:dr: In order to get drivers working on Windows 8.1, some incorporation of Windows 10 functions will most likely be needed. There are some hardware vendors still with 8.1 support, but some of the new laptops come with that hardware tied into other components meaning the standard driver will not work. Installing 8.1 is as simple as making the media as UEFI in Rufus, installing it with USB Mouse (and keyboard if on desktop), deleting Intelpep.sys, and finishing it up.