On Windows when IsReadOnly state changes to true, the control state does not change, you can still edit the text.
Example:
<VerticalStackLayout >
<Switch IsToggled="{Binding IsReadOnly,Source={x:Reference EntryShareName}}"/>
<telerik:RadEntry x:Name="EntryShareName"
FontSize="14"
Placeholder="Enter text here"
PlaceholderColor="#99000000"
AutomationId="radEntry">
</telerik:RadEntry>
</VerticalStackLayout>