This week the latest Ubuntu release came out, Ubuntu 21.10 "Impish Indri". As with all releases since 2016, IBM Z is one of the supported architectures. General release notes for Ubuntu 21.10 are available: Impish Indri Release Notes
But as Frank Heimes details in his post on the Ubuntu on Big Iron blog: Impish Indri is out - another primate escaped! aka Ubuntu 21.10 got released!, there are many IBM Z (s390x) specific changes of note, including the following (paraphrased from his blog post):
And much more!
Be sure to read the full blog post for details about support, upgrade path, and links to downloads and other resources.