two bone constraint aim Ik, parents Ik missing, the most important one 
We have Aim IK and Two Bone IK ready to use:
Also, Flax support custom anim nodes for more complex scenarios:
(there are also Anim Graph Function to make some reusable chunks of anim graphs)
There is also direct access to skeleton bones read/write so you can modify bones in graph.
it’s not about bones, how one can implement parent constraint ?
I guess we don’t have this. If you want to attach object to a particular skeleton node you can use Bone Socket (eg. to plug gun or torch into player hand). If you provide more detailed description maybe we could work on adding specific node to handle this.
I didn’t knew about bone sockets, anyway because in Unity you have a parent constraint, that makes you attach the item to the player bones, you can add more bones as you like, you just set the weights, and integrate it in the animation built in
In this case Bone Socket is a way to go: