There seems to be a new opensource solution based on a WPF RichTextBox. The only caveat is it in the core only supports STAThreaded applications and in order to use in a i.e. ASP.net you need to call it in a STAThread (but there is a sample for that in the writeup).
For use in VSTO add-ins this is confirmed to work (ie. Outlook RTFBody)
Nuget:
https://www.nuget.org/packages/MarkupConverter/
Project:
https://github.com/figuemon/MarkupConverter
Writeup:
https://code.msdn.microsoft.com/Converting-between-RTF-and-aaa02a6e
