Are there examples of other production grade container runtimes?
Are there other implementations

Yes. The Container Runtime group of the Cloud-Native Computing Foundation has a list of runtimes:

They include:


Containerd
CRI-IO
Firecracker
gVisor
Kata Containers
LXD
Nabla Containers
Pouch
runc
Singularity
SmartOS
Unik


Do they have different approaches

While they cannot all be compared with each other directly, they all offer a way to provide a container runtime. In some cases, this runtime leverages the kernel, in others, they are in userspace. While some are explicitly proprietary, others are explicitly Open. Some are supported by large corporations (Google, IBM, Amazon, Alibaba), others by foundations or open source communities.

All of these are used in production, but perhaps vary in their popularity.