They have banned the use of VPN APIs to tunnel traffic through a filtering proxy. Apps such as AdGuard Pro used to use this to perform whole-device adblocking.
VPN APIs is for VPNs, not ad blocker apps. Apple is “sandboxing” applications, so apps have limited ability to interfere with each other or alter the overall workings of the phone. Which is a good thing, one app should not see other app's traffic.