If I were running a volunteer project, I would be dumping thousands a month into top-tier hosting across multiple datacenters around the world with global failover.
The alternative is to make big changes to your build system and packaging to compile N different versions of the executable/library. There's no easy way to just add a compiler flag that means "use AXV512 and generate SSE2 fallbacks where necessary".
The people that want to keep running new third-party binaries on 12+ year old CPUs might want to work with the compiler teams to make it feasible for those third parties to automatically generate the necessary fallback code paths. Otherwise, there will just be more and more instances of companies like Google deciding to start using the hardware features they've been deploying for 15+ years.
But you already know all that, since we discussed it four months ago. So why are you pretending like what you're asking for is easy when you know the tools that exist today aren't up to the task?
I finally just upgraded my 9 year old computer with an i5-6600k to a Ryzen 9 5950x because I wanted to be able to edit home videos. I already rarely even used 1 core on the old CPU, the new one is 7x more powerful, and it's an ebay part from 5 years ago. I don't foresee needing to upgrade again for another decade. I probably would've been good for another 15-20 years if I had upgraded to a DDR5 platform, but RAM prices had already spiked, so I just swapped the motherboard and CPU.