Drifty Co. introduced Ionic in the year 2013. It released the first alpha version of the Ionic framework in November 2013. The latest release of Ionic is Ionic v4+ in 2019. We can understand the history of the Ionic framework from the below image.
Historically, Ionic 1 was introduced to create mobile apps with angular 1. The Ionic 1 was released at the end of the year 2013. Initially, it does not use a web component. It uses angular one directives to essentially give you a custom angular component which only runs inside the angular app.
Ionic 2 was introduced in the year 2016. It is continuing with the previous version by giving the same for angular 2. Angular 2 is the new version of angular, which is entirely different from angular 1. Thus, Ionic 2 again gives only the angular component, not the web component. So we can only use Ionic 2 in angular 2 application.
Ionic 3 was introduced in the year 2017. It is continuing with the previous one by adding some new features. This version enhances the Ionic framework but still focus on angular, which was angular 4 at that time. The Ionic team decides that the Ionic 3 is not optimal for future purposes because not all the people are using angular. It does not allow to build a native mobile app or allows to use of vue.js, or react.js framework, you were limited to angular only.
Every new version of Ionic framework releases in every six months with some enhancement. These new versions do not change your entire concept, which you had previously learned in the Ionic.