Starting with the new Alpha 3.11 patch, Star Citizen officially requires AVX instruction support if you want to play the game. This means all CPUs predating Sandy Bridge and AMD's Bulldozer architecture are no longer supported. Even worse, all of Intel's modern Celeron and Pentium CPUs (including the latest generation) can't run the game as they do not support AVX instructions, either.
That's a problem that's artificially created by Intel's segmentation practices. There's very little reason not to include AVX instruction support on these budget chips as AVX has become more popular since its creation. However, that should change in the future as Intel is adding support for AVX to its forthcoming low-power chips.
In a Spectrum post from CIG explaining the new AVX requirement, many people were very disappointed. Apparently, there's a decently large player base that runs Star Citizen on lower-end hardware, from Core 2 Quads and first-gen Core i7s to modern-day Pentiums and Celerons. These people will have no choice but to upgrade to modern hardware if they want to continue to play the game.
But this shouldn't be a big surprise: The scope and scale of Star Citizen are utterly gigantic, and the game is incredibly CPU-intensive in it's busiest areas. Adding AVX instructions to the engine is probably a good move by CIG to help increase CPU performance.
For now, there's no word from CIG on creating an SSE compatibility layer for AVX instructions on older CPUs, but the chances of that are probably nill. There's little incentive for CIG to implement a compatibility layer as the game requires high-performance systems in the first place to run the game at playable frame rates (and all of today's CPUs that qualify support AVX instructions). Plus, the player base that runs the game on 10-year-old hardware is very small and will get smaller as time goes on.