Thanks. I was also missing the newer version of this root.

More relevant information here: https://developer.apple.com/documentation/security/updating_mac_software I think there may be some poor behavior in Xcode that can cause this situation to happen because I run in to this sometimes without doing anything too exotic.

Assuming you just want to use it in 2D, MTKView is built on CAMetalLayer which is available on visionOS. Might be they forgot to put it in. It wasn't on tvOS originally either, but added later. It is easy enough to recreate it from scratch. Lots of examples online for those that needed more control. I was able to build my own version on visionOS with no issues.

If it helps, all browsers including WebKit can cache proxy information. You may need to take care you are not resuming a previous session. The site cache you are loading from might show stale information from before the proxy was set. WebKit runs in another process that might be caching some of this stuff. I'm not sure if deleting the app is enough to evict this cache. With proxy auto-configuration, the PAC may be cached too and pick up an old version. Wipe the device if you can for testing.

Additionally if running in the simulator, you need to provide environment variables in the run settings to use the proxy: http_proxy and https_proxy