ConnMan is a connection manager which integrate all critical networking components. It provides a smart D-Bus API to develop an User Interface. It is plugin oriented and all different network stacks are implemented in different modules.
Connection sharing (aka tethering) is using Netfilter to setup NAT masquerading. So it is a simple usage.
Switching to nftables
Application connectivity is a more advanced part involving Netfilter as it makes a use of statistics and differenciated routing. For example, in a car, service data must be sent to manufacturer operator and not on the owner network.
To do so a session system has been implemented. Application can be modified to open a session to ConnMan. This allow to define a per-session policy for routing and accounting.
ConnMan team wanted to use a C API to do rules modification but this was difficult with iptables and xtables. This is not an official API so it is subject to bugs and change.
ConnMan team has then switch to nftables and is currently working on stabilizing nftables to ensure the acceptation of the project and of the maintainability of their solution in the long time. This work is not yet upstream but there is good chance it will be accepted.