ifconfig is deprecated, and has been so for quite a number of years. The new kid in town is the ip command, which can configure IP's, routes, and everything associated with networks.
You can install ifconfig with sudo apt install net-tools, if you absolutely need to have it. If not, start learning ip.
In short, it is removed because you should not use it. It has mediocre IPv6 support, the ip command is a better replacement. I find sources saying ifconfig was deprecated back in 2012, so I'm not surprised it's finally removed.
You may be able to install it on 18.04, but on future releases it may disappear entirely. I would consider it not being installed by default as a warning shot: learn ip.
Thus ifconfig is probably in a special class. In previous versions of Ubuntu it was included in minimal installs. It is now being phased out, and you explicitly have to install it. This can not be taken to indicate that any other packages are missing.
