The autocad file format can be saved as a previous version but the civil 3d objects will remain as 2010 or which ever version they were saved in unless they export to autocad format in which case the civil 3d objects will be exploded.
If you really have to go backwards, I think XML is the best option but it only does the raw objects, not the views etc.
You'll need to have your subs export the xml before they send the drawing across to you.
Best option is to have them work in the same version as you. Good luck on that one.
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Andrew Puller
Maitland, NSW, Australia
Windows 10 Enterprise 64bit
Intel core i7 11800 @ 2.30 GHz with 32GB Ram
Civil 3d 2021