Are there Realease Notes when a new Release is published?
You should find release notes for all versions linked from https://civicrm.org/blog/tags/release (this page is linked from the download page). Maybe they haven't added in the latest yet: 5.18.0 not there either

https://lab.civicrm.org/dev/core/blob/master/release-notes.md links to all the release notes including 5.18.0 and 5.18.1