The reason there isn't a DataTemplateSelector for the GroupHeaderTemplate is because the BindingContext of the GroupHeader is always the same, there's nothing to select on (i.e. it will always be an instance of type Telerik.XamarinForms.DataControls.ListView.GroupHeaderContext).
Customize Header Based on Group
That being said, I'm assuming you're ultimately looking for a way to customize the contents of the template based on different values for the group Key.
This can be done with other styling approaches like converters or DataTriggers because the GroupHeaderContext gives more than just the text you see in the header (that's the Key.ToString()).
If you would like to get further assistance, I recommend opening a support ticket. You get full support during your trial. A Support Ticket guarantees a response from the Telerik UI for Xamarin team within the ticket's time-frame.
We do our best to answer forum threads within a reasonable time-frame, but this is limited by available resources (i.e. current support load) and a forum reply can take a few days because support tickets always come first.
Note: I see you currently have trials, if your employer has a purchased version make sure they assign you as the Developer. This will give you full access to their Support plan and products, but they still own the license. It only takes 20-30 seconds via the Manage Licensed Users page.
Lance | Team Lead - US DevTools Support
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