Why has no new version of calico bird been released in recent years?

On github, the latest version of calico bird was released on Jan 20, 2018. No new version has been released in recent years. What is the version release plan of calico bird? Or why hasn’t a new version been released recently? Thank you.
Calico Bird Github Address: GitHub - projectcalico/bird: Calico's fork of the BIRD protocol stack

Its not intended to be a repo that has standalone releases - its a component of Calico that gets imported into calico-node.

We would expect anyone who wants to use Bird standalone to use the upstream version

Anyone who wants to use it with Calico, will find it already included in calico-node.

If I only use bird in calico node, which version of bird do you suggest we use?
Use the bird in this repository?(GitHub - projectcalico/bird at v0.3.3))
Thank you.

What’s the use-case here? i.e. Why would you use only bird from calico-node?

We use the compilation method provided in Calico to compile images. Bird is used when compiling Calico nodes.

Is there a particular code change in the bird repo that you think isn’t included in the latest calico-node image?

In calico/metadata.mk at b575532df5fdb508552ce70b14c50fa36b8f40e1 · projectcalico/calico · GitHub, we see that for Calico v3.22, the BIRD_VERSION is set to:

# The version of BIRD to use for calico/node builds and confd tests.

You should use whatever BIRD version matches the Calico release you’re trying to build.