The nTB MBP is still a mid-2017 model.
This model uses a 15W TDP CPU with Iris Pro graphics. Intel has not released 8th-gen CPUs that fit this specification (i.e. Iris Pro Graphics) - i.e. there is no direct successor to the chip on the nTB. Touch Bar models use 28W TDP CPUs. I don't think we'll see that chip in the near future, so Apple will either leave it as is or discontinue it.
Do have in mind though that the nTB MBP is still significantly faster than the new MacBook Air, both in raw power, sustained performance and (especially) graphics - the downsides being the older generation keyboard and lack of Touch ID.