Well, /net is entirely disabled by default as of recently, so this entire method is no longer applicable. However, since you asked, here is some useless inform

Well, /net is entirely disabled by default as of recently, so this entire method is no longer applicable. However, since you asked, here is some useless information: With /net or remote filesystems in general (NFS and SMB), the network accesses are performed by the kernel directly, rather than by the application using networking syscalls. Therefore, sandboxing network access from specific applications won't affect it. Big Sur doesn't actually have a permission dialog for network access. But TextEdit does use the (long-existing) App Sandbox system, which is based on applications statically declaring permissions they need. Since TextEdit doesn't request a networking entitlement, it's prohibited from accessing the network directly; as I said, that doesn't include remote filesystems.