Even though this isn't a good time to buy a GPU, I'm still thinking about it, and rather than ask about a particular model, I figure I'd ask about the cheapest one that satisfies certain minimum requirements, which are as follows:
* Must be a discrete GPU that can fit inside a PCIe x16 slot. (Anything that's part of a CPU would require a new motherboard and RAM as well.)
* Must support OpenGL 4.6 and Vulkan 1.2 under Linux with open source drivers. (The likes of LLVMpipe and other software rasterizers don't count; this has to be done on the GPU, or otherwise there's no point.) Also, the support must be good; noveau is not acceptable for this purpose.
* This support must be available on Linux, with the upstream Linux kernel (5.10, which is the current LTS), and xorg and mesa. Also must work under wayland. (Note that this criterion excludes nvidia drivers.)
* For purposes of this topic, price and compatibility are the only thins that matter; performance is not a concern.

So, what would the cheapest GPU be that meets these requirements? (Alternatively, what's the oldest GPU that does?)

Looks like you're going to need the AMD Radeon RX 5000 series, so only the previous generation, if you want OpenGL 4.6. Older than that was only 4.5. AKA, these are still "new" and pricey, since the 6000s are basically unavailable.

The mini-generations (RX Vega) between RX 500 and RX 5000 supported it, but they were rare and high priced cards.

didnt have much option but to buy one of these about a month ago (managed to get it for £80 with free delivery, so not massively gouged compared to similar / the same card on other sites where it was £100).
https://www.asus.com/uk/Motherboards-Components/Graphics-Cards/Phoenix/PH-550-2G/ (its a 2gb rx550).

has open gl 4.6 and vulkan 2 or 2.1 (the latter only with the latest drivers i think). should work fine on linux as its amd but still havent created a partition to test.

managed to find more details on my system. hope this is of use.

The RX 500s are listed as only supporting OpenGL 4.6 in Windows, 4.5 in Linux. 4.6 in Linux seems to require the 5000s. From what I can find. I may be wrong. Probably am. Hope I am. But the docs I've seen indicate that for some weird reason.

ah... poo!

Edit: i need to make a linux partition before the end of the week anyway so will see what i can find out once i have that done.

Basically, rx200 and up, but realistically rx400 and up.

https://en.wikipedia.org/wiki/Radeon_RX_Vega_series#Radeon_features

I have rx570 8gb version, here is the glxinfo: https://pastebin.com/zZm1Pu39

Well, it fits inside pcie x16. I paid like $120 for it in 2019 if I remember correctly.

And here vulkaninfo: https://pastebin.com/DC63xkGg