We've now released (optional) support for hosting on ubuntu 18.04. Until recently our hosting images were based on Ubuntu 16.04, the previous long term support release of Ubuntu, but now we've completed the necessary integration work and testing to support 18.04 as well.
In practical terms there aren't a lot of differences between the versions..The versions of core apps have all be updated (PHP versions are updated independently of the operating system in Idealstack). Probably the biggest difference is the use of a new version of Apache . There may be minor performance differences between the versions although we don't see any in our testing.
Your existing sites won't change but new sites you create default to 18.04 and you can choose to update your existing sites by editing the Hosting Plan. Ubuntu 16.04 continues to receive security updates until January 2021 so you don't need to rush to do this, we will release more updates about how we will automatically port your sites to 18.04 closer to the end of life for 16.04