The Linux kernel is not usable as a security boundary, so anyone who wants to do "shared hosting" and not be hacked needs to use something else, like gVisor or firecracker VMs

The only important system that uses it as a security boundary is Android and there is mitigated by the fact that APKs need user approval, plus strict SELinux and seccomp policy plus the GrapheneOS hardening, and in this case the mitigations succeeded (https://discuss.grapheneos.org/d/35110-grapheneos-is-protect...)