I haven't implemented a production Notebook Server environment due to the lack of support for Docker Community Edition on RHEL. However at 11.2 I see that it now supports Docker Community Edition on RHEL 8. We recently upgraded to 11.1 and were planning on upgrading again a 11.3 since its the long term release.
I know you can have different version of ArcGIS Server (as long as they are not the hosted server) federated with Portal, is it supported to have a newer version of ArcGIS Notebook Server federated with an older version of Portal?
No, ArcGIS Server (including Notebook Server) cannot be a newer version than that of Portal. You would need to run through an entire upgrade to 11.2 to be able to take advantage of that system support for RHEL.