> The variable length vectors are probably one of those ideas that sound good on paper but don’t work that well in practice

I don't understand this take, you can still querry the vector length and have specialized implementations if needed.

But the vast majority of cases can be written in a VLA way, even most advanced ones imo.

E.g. here are a few things that I know to work well in a VLA style: simdutf (upstream), simdjson (I have a POC), sorting (I would still specialize, but you can have a fast generic fallback), jpeg decoding, heapify, ...