[kemotep]

I know there are features to hotpatch the kernel, don’t know if that’s limited to Red Hat or Ubuntu or if something like Arch can do it too.

If individual packages or services have to restart does not qualify for your requirements then I don’t think any system could truly provide that without having it run multiple instances of each service/itself.

[jacquesm]

I've used QnX and a home grown clone that could do this. In... 1991. Erlang can do it too.