AFAIK, the UEFI spec imposes no requirement that (non-hotplug) devices be re-initializable after you've already initialized them once. Devices are free to take the "ExitBootServices has been called" signal from EFI and use it to latch a mask over their ACPI initialization endpoints, and then depend on the device's physical reset line going low to unmask these (as the device would start off in this unmasked state on first power-on.)
Devices are also free to have an "EFI-app support mode" they enter on power-on, and which they can't enter again once they are told to leave that mode (except by being physically reset.) For example, a USB controller's PS2 legacy keyboard emulation, or a modern GPU's VGA emulation, could both be one-way transitions like this, as only EFI apps (like BIOS setup programs) use these modes any more.
Of course, presuming we're talking about a device that exists on a bus that was designed to support hotplug, the ability to "logically" power the device off and on — essentially, a software-controlled reset line — is part of the abstraction, something the OS kernel necessarily has access to. So devices on such busses can be put back in whatever their power-on state is quite easily.
But for non-hotplug busses (e.g. the bus between the CPU to DRAM), bringing the bus's reset line low is something that the board itself can do; and something that the CPU can do in "System Management Mode", using special board-specific knowledge burned into the board's EFI firmware (which is how EFI bring-up and EFI ResetSystem manage to do it); but which the OS kernel has no access to.
So while a Linux kernel could in theory call ExitBootServices and then virtualize the API of EFI boot services, the kernel wouldn't be guaranteed to be able to actually do what EFI boot services does, in terms of getting the hardware back into its on-boot EFI-support state.
The kernel could emulate these states, by having its native drivers for these devices configure the hardware into states approximating their on-boot EFI-support states; but it would just be an emulation at best. And some devices wouldn't have any kind of runtime state approximating their on-boot state (e.g. the CPU in protected mode doesn't have any state it can enter that approximates real mode.)
AFAIK, the UEFI spec imposes no requirement that (non-hotplug) devices be re-initializable after you've already initialized them once. Devices are free to take the "ExitBootServices has been called" signal from EFI and use it to latch a mask over their ACPI initialization endpoints, and then depend on the device's physical reset line going low to unmask these (as the device would start off in this unmasked state on first power-on.)
Devices are also free to have an "EFI-app support mode" they enter on power-on, and which they can't enter again once they are told to leave that mode (except by being physically reset.) For example, a USB controller's PS2 legacy keyboard emulation, or a modern GPU's VGA emulation, could both be one-way transitions like this, as only EFI apps (like BIOS setup programs) use these modes any more.
Of course, presuming we're talking about a device that exists on a bus that was designed to support hotplug, the ability to "logically" power the device off and on — essentially, a software-controlled reset line — is part of the abstraction, something the OS kernel necessarily has access to. So devices on such busses can be put back in whatever their power-on state is quite easily.
But for non-hotplug busses (e.g. the bus between the CPU to DRAM), bringing the bus's reset line low is something that the board itself can do; and something that the CPU can do in "System Management Mode", using special board-specific knowledge burned into the board's EFI firmware (which is how EFI bring-up and EFI ResetSystem manage to do it); but which the OS kernel has no access to.
So while a Linux kernel could in theory call ExitBootServices and then virtualize the API of EFI boot services, the kernel wouldn't be guaranteed to be able to actually do what EFI boot services does, in terms of getting the hardware back into its on-boot EFI-support state.
The kernel could emulate these states, by having its native drivers for these devices configure the hardware into states approximating their on-boot EFI-support states; but it would just be an emulation at best. And some devices wouldn't have any kind of runtime state approximating their on-boot state (e.g. the CPU in protected mode doesn't have any state it can enter that approximates real mode.)