It looks like the cutoff this year is Metal support. I can't think of a similar cutoff for the near future, so I bet your Best Damn MacBook Pro Ever will continue to get updates for a while.
Interestingly, up to now all Metal compatible GPUs on Mac have been treated as one family with the same feature-set, but this year Apple is introducing new features for all Mac Metal GPUs as GPUFamily1_v4 and also offering exclusive features for newer Metal GPUs in GPUFamily2_v1. No idea which GPUs qualify as GPUFamily2, but it probably corresponds to Direct3D feature level 12 compatible GPUs like AMD GCN 2 and Intel Skylake or better. Hopefully it'll be a few years before Apple actually requires Metal GPUFamily2.