I have a kinda odd question. In theory, would it be possible for a new Mac (Lets say a fancy new MBP) to downgrade from macOS High Sierra to a OS X 10.4.11 Tiger Intel build? If so, what would be the complications of doing so, and would doing so make it irreversible?
Golden Rule: No Mac can run an OS older than the OS it shipped with.
The hardware drivers do not exist, as the hardware wasn't invented at the time.
From Snow Leopard Server 10.6/Lion 10.7 you could virtualise it in Parallels etc, but nothing before that.
Probably not, but there is a slim chance.
Tiger would have supported Code 2 Duo (C2D) machines since some were introduced in 2006 during Tiger's reign. The last version of Tiger was released around November of 2007 by which time Apple would have been shipping only Leopard (10.5) with new machines. All of the C2D machines presumably have a 64-bit EFI whereas the older "Core Duo" (note no '2') were 32-bit.
The one thing that is a fairly low-level "make or break" factor on Intel hardware is the combination of CPU and the Intel "chipset", i.e. the main support chip(s) that are required for the CPU. An example from the C2D days is the Intel P45. Simply put, this is where you get the interface to memory, PCI lanes, USB, etc. Operating systems need to know how to speak to this more so than the CPU since this is all the distinct services for the x86 platform.
So while the chipset probably didn't change much between the C2D machines, what would have changed was the GPU they shipped with it. 2006/2007 had a mixture of nVidia, ATi/AMD and Intel graphics. I believe a 2009 MacBook shipped with an ATi Radeon 9000 series which support might be in Tiger for, but most likely not since the PCI Express ID signatures would probably get missed and thus you'd have no video. Unlike Windows, OSX is very picky about GPU drivers and if it doesn't recognize the PCIe ID of a GPU it won't fail-over to a basic VGA mode like Windows would.
That being said, if you can somehow convince the Mac to install Tiger, you could look into editing the plist files that identify the list of GPUs and try to pick one that is "close enough". However, this would not be easy and requires rebuilding kext plists and all sorts of magic that is not trivial.
The only other thing that might catch you is that the Tiger installation scripts might not recognize the motherboard/Gestalt ID of the newer machine and could stop right there. Obviously Hackintosh folks got around this, but you'd have to dig back to find notes from the Tiger days.
In short, I'm guessing that the OS would probably work on the 2009 MacBook, but whether or not you get a display is the place one would get hung up.
No, you can‘t. Older OSX/macOS versions lack the drivers required to run on newer hardware. Usually you can‘t go back before the macOS Version your shiny new Mac was delivered with.