Creating a folder in the root of the filesystem results in "Operation not permitted" The following command:

$ sudo mkdir /AppleInternal
and entering my password when prompted,

results in:
mkdir: /AppleInternal: Operation not permitted

Is this because of System Integrity Protection? I'm sure I've done this previously with no issues. I'm on High Sierra (10.13.4).

btw, the reason I want to create that folder is so I can run the iOS Simulator in split screen: https://stackoverflow.com/a/46887389/368085
This is because of SIP, but only with the name ‘AppleInternal’. You can create other folders and files in the root, but the name AppleInternal is restricted. You can neither create a folder with the name AppleInternal, nor create a folder with a different name and rename it AppleInternal.