You can download RVT 2021 Sample Files from here:
https://knowledge.autodesk.com/support/revit-products/getting-started/caas/CloudHelp/cloudhelp/2021/...
In Archicad, you can place a RVT file as Hotlink, so I tried it with one of these sample files. I could not, and a message came up that only Revit 2015 to 2020 files are supported.
When exporting the 3D Viewpoint as an RVT file, again, there is info that the exported file will be in RVT 2020 format.
I think the reason for this is that the ODA solution does not support Revit 2021 yet, so GRAPHISOFT has to wait till ODA supports Revit 2021 before they can implement it in Archicad 24.
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