It simply means that the "OnPlayerInit" hook has been replaced by "OnPlayerConnected" in a Rust/Oxide update and the deprecated hook will no longer work after the date given. I notice the plugin page states that the plugin will be removed urging people not to use it. Bare in mind any fix might be temoprary as a further Rust update may break the plugin again. I assume the plugin authour is no longer maintaining this.
Still, for now you should be able to fix that message and keep the plugin working beyond April 1st by doing this:
Open up the plugin file in a text/code editor (I recommend Notepad++), go to line 130 and change it from:
OnPlayerInit(players[i]);
To:
OnPlayerConnected(players[i]);
Then go to line 158 and change it from:
private void OnPlayerInit(BasePlayer player) => InitPlayer(player);
To:
private void OnPlayerConnected(BasePlayer player) => InitPlayer(player);
I don't use the plugin, but that should fix it for you until a further Rust update potentially breaks it again.