Will the new changes to Android 9 cause the VPN based firewall apps become obsolete?
Have a read of the commit on googlesource here. It states


  Files in /proc/net leak information. This change is the first step in
  determining which files apps may use, whitelisting benign access, and
  otherwise removing access while providing safe alternative APIs.


[....]


  
  Labels /proc/net/{tcp,tcp6,udp,udp6} as proc_net_vpn due to existing
  use by VPN apps. This may be replaced by an alternative API.
  Audits all other proc/net access for apps.