I noticed here https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/#create-a-list-file-for-mongodb that we list Ubuntu 14.04 and 16.04 but not the latest LTS version, 18.04 (released in late April 2018).

As far as I can tell, you could update the Ubuntu 16.04 heading to say Ubuntu 16.04 / 18.04. I am setting up 18.04 now and just used the 16.04 instructions.