Funny thing is, I can find all the RAM I want for my MP 1,1 on Ebay, and it's not even all that expensive unless you insist on true "Mac Pro" RAM(with gigantic heatsinks). Heck, it didn't even cost me that much to drop in 16gb with the correct heatsinks-it was less than it would have cost me to upgrade my mid-2012 MBP to 16gb if I hadn't bought the computer with that much pre-installed. The early Mac Pros use DDR2 registered FB-DIMMs, which are otherwise pretty much the domain of(now antiquated) servers so ram on the whole for them is cheap.
I get tired of hearing about supposedly "hard to find" RAM for older computers. SD-RAM, DDR, and DDR2 are quite plentiful and are inexpensive on Ebay. I tend to buy SDRAM in bulk because of the number of G3/G4 era computers I have, and can usually get my choice of LD 256mb PC-100(needed for Powermac G3s and one particular G4 model) as well as LD 512mb PC-133 for about $5/stick if I shop around. SO-DIMMs run the same price, although it requires being a bit more vigilant if you want LD 256mb sticks. I maxed a G5 not too long ago for around $40.
The hard to find and expensive argument isn't really true until you get into pre-G3 stuff. I have both a Kanga and 3400c. These are very similar laptops, with the former having a G3 and the latter a 603e. They require a special, proprietary upgrade card. I was very lucky to get the rare 128mb card with my Kanga, while my 3400c only came with a 32mb. 128mb cards are available from some specialty suppliers, but are well over $100. Many other Macs of the era used 168 pin DIMMs, and aside from ram hoardes(I have combined a few that I got from old Mac guys) the only reliable source is the 128mb modules available at OWC. In the last few weeks, they have doubled their price on these to $20 each.
@LightBulbFun had to hunt hard for the ram for his PowerMac 4400, and when he found it on Ebay he had it shipped to me since the seller wouldn't ship to England(as luck would have it, I actually found an additional 32mb stick for him in one of my ram drawers).
I agree that some of the cut-offs are baffling. The late 2008 Al Macbook is the strange one, as it uses the same GPU and and very similar(DDR3) architecture to the supported late '09 Macbook. In fact, the GPU in the Al Macbook has double the VRAM of the late '09 white iBook. The other '09 Macbooks share a common link in the 9400M also(actually it's in a lot of the now deprecated computers) although admittedly they are DDR2.
@MagicBoy 's Mini is telling about what we can expect, though. I downloaded Sierra a little while ago-in fact I've been meaning to sign up for a developer account for a while and this seemed as good a time as any to do it-although I haven't spent much time playing in it as I put it on my main MBP in a second partition and needed to get some other work done. Given the discontinued list, it looks like nVidia 8xxx series GPUs may be done. If that's the case, my '08 MBP will stay at El Capitan and my Mac Pro may need a new video card.
BTW, the fact that the new OS is free is irrelevant. People have been hacking OS X to run on unsupported hardware for ages-since Apple cut off a handful of Macs(specifically beige G3s and Wallstreet Powerbooks-anything without built-in USB). Tiger was hacked to run on Macs without built-in Firewire, and installing Leopard on an AGP-based G4 is trivially easy. Bear in mind that this was back when a new version of OS X cost real money-usually around $130 or so. Heck,
@LightBulbFun was even able to make Tiger run on 603 and 604 series CPUs.
The only time it really hasn't happened is when a major shift in architectural requirements has occurred. This happened with Leopard, which requires a G4, Snow Leopard requiring an Intel processor, and Lion requiring full 64 bit processors. Aside from that, there's usually a way, even though in some cases you can only maybe stretch generation past the newest official one. My Blackbook(late '07) is running Mountain Lion, but I've been strongly encourage to not attempt beyond there.
By the way, there is a program called PCI Slot Utility that is only applicable to Mac Pro 1,1 and 2,1s. Basically, it lets you adjust the bandwidth allocated to each slot. It's sort of "hidden" but will pop up on a 1,1 if you install a new PCIe card. Despite this officially not having been needed since 10.7(the last OS these computers supported) it is still is present in El Capitan. Just something to think about...
BTW, El Capitan offers a fair number of features that my early '08 MBP(officially supported) can utilize. They just don't work in El Capitan.