Is there any way to automatically clean-up or remove nodes stuck in a registering state in the Rancher U/I after a certain timeout period? I notice with auto-scaling that sometimes a node doesn't join, and gets cleaned up by the ASG/Cluster Autoscaler but is always still in the rancher U/I and has to be manually removed.

for the Rancher agent you can run

rancher-system-agent-uninstall.sh

on the node you want to remove. Also there are scripts depending which kind of cluster you are installing. I know for RKE2 there are:

rke2-killall.sh
rke2-uninstall.sh

After that is everything on the node stopped and removed. Then just remove the node from the Rancher GUI.

I was looking at more of a back-end setting that makes rancher terminate a node automatically if it takes too long to register. (Times out)

i dont know, it did that on its own after 5 minutes for me