Can you expand on that? As far as I'm aware that's not true (anymore). It runs quite fine on AWS which uses older NVIDIA cards and I know several people use it on older-gen GPU-enabled MBPs.
In my experience, the architecture supports cards with shader model >= 3.0. Occasionally a commit will break the support (eg https://bitbucket.org/eigen/eigen/commits/a19653b8035d8ace53... was required earlier this year) but this is a function of the speed of development and usually straightforward to fix.
Yeah, those are just cards we know that work: we don't have all possible GPUs to test our 20+ changes a day on, so we can't formally guarantee it will work on older cards, but in general we try our best to keep it all working for older and even newer cards.
