It's good to hear that you have managed to get data binding with RadRichTextBox working. There should not be any issues with the data providers in code-behind. One thing you should have in mind is that if their DataContext is changed and they are not added to the visual tree, they will not be notified. Therefore, if you will be changing their DataContext, you must add the providers to the visual tree. This should not present a concern, as they have no visual representation and will not have an effect on the layout or the performance.
As for RadHtmlPlaceHolder, you are right, it has been obsoleted and RadRichTextBox is the control that has the closest features. You should have in mind, though, that HtmlFormatProvider (which HtmlDataProvider uses internally) cannot maintain the HTML input on import/export, i.e. once you import an HTML string/stream, it is no longer kept in memory. A RadDocument is created out of it, following the structure specified in the HTML file as close as possible. On export, an entirely different HTML is generated, which can have little in common with the initial input. Hopefully, you will find the way RadRichTextBox works suitable for your project.
All the best,
the Telerik team
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