Microsoft did this intentionally because they are a bunch of **** who don't care about how you want to use your computer. It's their way or the highway.
https://docs.microsoft.com/en-us/windows-hardware/customize/desktop/customize-power-slider
This is currently the best option:
http://en.community.dell.com/support-forums/mobile-devices/f/4586/t/19578910
Use Regedit and change the registry key below:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power
Change the value of CsEnabled from 1 to 0 and reboot.
This disables connected standby, which also disables the slider in the battery task tray icon panel, and restores at least 1 of the power profiles to the [control panel -> hardware & sound -> power options -> power plans] UI.
For me, the fix reverted after a reboot and the slider came back.
**** microsoft