You still choose to use Linux LTS release 18.04. That comes with the software packaged for that version. You should be fully safe when installing and using software provided with the distribution.
This is Linux, so it it certainly possible to install software in other ways. That can be through installing .deb packages from elsewhere, adding PPA's, copying executables directly onto your system, or ultimately, by compiling software yourself.
Is it safe? You are on your own. If you know what you do, if the sources you consult are reliable, if you proceed the correct way, etc., you will probably not break your system, or at least not badly.
Specifically on Xmonad: if you did not find a PPA prepared specifically for Ubuntu 18.04, it may not exist. If there is a .deb file, you could try installing it with apt. If apt is able to resolve the dependencies, installation will proceed. If it doesn't, you will have error messages, and installation will not proceed. However, your package management system will not be broken. So that is relatively safe to try.
Be more careful with PPA's. If you add PPA's not designed for your Ubuntu version, your package management may break. And that is sometimes difficult to resolve again.
For sure, there is source code of Xmonad, so skilled linux users can compile. And obviously, you could upgrade the Ubuntu system to have a more recent version of Xmonad.
