I don’t think it’s wise to do this, for the reasons outlined above. If iptables is being deprecated in distributions, there is likely good reason for it.
Do you have any proof to suggest that users have not updated because iptables is no longer the default firewall implementation on their system? By your own admission, you only discovered this post-upgrade, so this can’t even be anecdotal evidence.
OSMC can’t, and won’t, give you an error telling you to post on the forum when running a command from the CLI. We don’t have anything built-in to do to that. It can only give you this error if there is a problem when updating via My OSMC.
I made our Debian Buster test release available for a month before releasing it as stable, and only did so when we had fixed a number of issues.
Unfortunately I didn’t have any reports about the change of behaviour regarding firewalling. I suspect some users made this change with aid of a quick Google search and didn’t bother to update the testing thread with feedback.
It wouldn’t be possible to rectify things now - as the update has been out for almost a month and has been deployed to almost all of our active users.
Deploying firewall rules via the command line is complex. It is quite plausible to make mistakes that can result in a compromised system. We recommend that users rely on an external firewall or even NAT if they are IPv4 only. Our goal is to provide a mediacenter with a full fledged Linux distribution underneath, but we cannot provide a supported migration path for every change made by a user. This is the downside (for us) of making the system so configurable.
Our goal is to provide a mediacenter that requires no command line use. If you do choose to use the command line however, we expect that you are able to run some commands. If you choose to make significant changes to the system, then we expect you to adapt those changes as the system upgrades. Otherwise - where should it end? It’s possible to install a desktop environment and office suite on OSMC, but hopefully you can appreciate that this isn’t something that we would have tested when working on the Debian Buster upgrade.
I think the real long term solution is to make firewalling configurable via My OSMC’s network interface with a simple UI.
Please don’t hesitate to let us know if there’s anything that we can do for you in the future.