Yes, this is a known issue. Many of the Noesis extensions listed here

https://www.noesisengine.com/docs/Gui.Core.ExtensionsTutorial.html

are not implemented in WPF/Blend. Not sure if all of them can be implemented.

If this is very critical for your development we can investigate how to bring Path Trimming to Blend.

Long term solution for this: we are working in our own editor, similar to Blend, but created with Noesis.

It isn't currently critical for us, although it would be useful.
Can I ask what the ETA is on your editor as this will have an impact on how we plan our workflow going forwards?
Also, it would be very helpful if you were to add notes in the documentation you mentioned, to identify which components are known to be non functional in WPF/Blend, as our designer lost a significant amount of time attempting to work out what they had done incorrectly before we realised that this was an issue with the extension itself.

There is no ETA for the editor right now, it is a long-term milestone (not happening this year or next one).

Sorry about the problem with your designer. We will improve the documentation.