So the code sample I linked to (on the master branch) is using the latest Angular, alpha.52, which changed all of the markup syntax to camelCase. If you are using alpha40 you should be using ng-model:
<ion-list radio-group [(ng-model)]="relationship">
<ion-radio value="friends">Friends</ion-radio>
<ion-radio value="enemies">Enemies</ion-radio>
</ion-list>
<div padding>
<code><b>relationship:</b> {{relationship}}</code>
</div>
I confirmed that is still working. However, we will be doing a release of Ionic 2 alpha 41 shortly which will require the camelCase changes.
See Angular’s changelog for more info:
<a name="6.0.0-beta.0"></a>
# [6.0.0-beta.0](https://github.com/angular/angular/compare/5.2.0...6.0.0-beta.0) (2018-01-17)
### Bug Fixes
* **animations:** fix increment/decrement aliases example ([#18323](https://github.com/angular/angular/issues/18323)) ([d2aa8ac](https://github.com/angular/angular/commit/d2aa8ac))
* **benchpress:** should still support selenium_webdriver < 3.6.0 ([#21477](https://github.com/angular/angular/issues/21477)) ([9b84a32](https://github.com/angular/angular/commit/9b84a32))
* **common:** set correct timezone for ISO8601 dates in Safari ([#21506](https://github.com/angular/angular/issues/21506)) ([05208b8](https://github.com/angular/angular/commit/05208b8)), closes [#21491](https://github.com/angular/angular/issues/21491)
* **compiler:** cache external reference resolution ([#21359](https://github.com/angular/angular/issues/21359)) ([e3e2fc0](https://github.com/angular/angular/commit/e3e2fc0))
* **compiler:** make `.ngsummary.json` files idempotent ([#21448](https://github.com/angular/angular/issues/21448)) ([e64b1e9](https://github.com/angular/angular/commit/e64b1e9))
* **core:** fix chained http call ([#20924](https://github.com/angular/angular/issues/20924)) ([7e3f9a4](https://github.com/angular/angular/commit/7e3f9a4)), closes [#20921](https://github.com/angular/angular/issues/20921)
* **ivy:** Add workaround for AJD in google3 ([#21488](https://github.com/angular/angular/issues/21488)) ([6af3672](https://github.com/angular/angular/commit/6af3672))
* **language-service:** Clear caches when program changes ([#21337](https://github.com/angular/angular/issues/21337)) ([43e1520](https://github.com/angular/angular/commit/43e1520)), closes [#19405](https://github.com/angular/angular/issues/19405)
* **service-worker:** properly handle invalid hashes in all scenarios ([#21288](https://github.com/angular/angular/issues/21288)) ([3951098](https://github.com/angular/angular/commit/3951098))
### Features
* **bazel:** allow ng_module rules to control whether type checking is enabled ([#21460](https://github.com/angular/angular/issues/21460)) ([cffa0fe](https://github.com/angular/angular/commit/cffa0fe))
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