I think the closest things done related to planes is References are not preserved after imports · Issue #1010 · Slicer/Slicer · GitHub which was prior to markups refactor, but includes work for a vtkMRMLMarkupsPlanesNode.cxx.
Related topics
| Topic | Replies | Views | Activity | |
|---|---|---|---|---|
| A user's proposal for Markups Module enhancements | 20 | 1702 | August 16, 2018 | |
| New Annotation for Slicer - Splines and Planes | 6 | 1012 | September 9, 2018 | |
| Markups for angle, line and closed/open spline | 5 | 1001 | January 11, 2019 | |
| Sandbox extension and Slicer version 5.8.1 | 2 | 52 | September 10, 2025 | |
| Fiducial Marker | 4 | 420 | September 2, 2022 |