As a result, it seems that 6th version should be released in March or April of 2018, and Angular 7 in September or October 2018. Each version is promised to be backward-compatible with the last release.
Angular 5’s promised features are: creating progressive web apps, build optimizer and accommodations for Material Design components.
The gap between Angular and Angular 2 was a few-years long. Google started to release new version 2 times in a year. Also, the 3rd version was skipped and Angular 4 was released.