Hello, I happened to find the same for an old iOS 18.6 Simulator runtime that always appeared after a reboot. Turns out that this was happening but I just didn't happen to notice, or I had the iOS 18.6 sim installed when using Xcode 26. But then recently I decided to remove the 18.6 sim from Xcode 27, then I came across the same issue.
Using xcrun simctl runtime delete will not work, as it will always be mounted back after reboot.. The runtime is located in the folder "/System/Library/AssetsV2/com_apple_MobileAsset_iOSSimulatorRuntime/" in a hexadecimal folder name.
Because the folder is protected by SIP, you cannot remove it normally. Boot to Recovery, then from terminal, identify the Data volume, and remove it. You can look up “macOS Xcode simulator runtime stuck in /System/Library/AssetsV2” online.