How can I make Ubuntu 18.04 / 18.10 desktop use Unity (be like Ubuntu 14.04)?

To install Unity you need to follow these steps:

• Open a terminal (Ctrl+Alt+T) and use the following command:

  sudo apt install ubuntu-unity-desktop
  

• At the installation time, you’ll be asked whether you want to switch to LightDM.

  

  A Display manager is what you see on the login screen.

  • If you want Unity like login screen: select lightdm
  • If you want to keep the default login screen in Ubuntu 18.04: select gdm3

  Just select the one you want and press enter to move ahead with the installation procedure.

  

• Once the installation is complete, restart your system. At the login screen, click on the Ubuntu symbol in LightDM or gear symbol in GDM.

  

• In here, you can see the option to use Unity desktop environment.

  

Source: It's FOSS

I think Tasksel is best and simplest way. Type:

sudo apt-get install tasksel

And then:

sudo tasksel

Uncheck GNOME desktop and check Ubuntu desktop. Then click OK. Reboot and you're done.