ATLauncher implemented a 'fabric' loader in May 2019, which can be used by modpacks on the platform. Loader support is documented on the ATLauncher's Wiki.

<version>
    <pack>
        <version>1.0.0</version>
        <minecraft>1.14</minecraft>
    </pack>
    <loader type="fabric" loader="0.4.8+build.154" />
</version>

• ATLauncher Wiki
• ATLauncher Discord