[cmeacham98]

Debian distributes compiled binaries. Thus, they have to either turn processor features on in all their binaries, or off (or distribute two sets of binaries).

[pabs3]

There are options for runtime instruction selection:

https://wiki.debian.org/InstructionSelection

[IntelMiner]

You're thinking of optimizing the code for a specific processor. Run-time codepaths that detect CPU features have existed since MMX and SSE

[sanxiyn]

Debian does distribute SSE-using binaries (with fallback) on i386 which detects presence of SSE at runtime using CPUID.