I also searched for this feature and I could find these tools:
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Node Editor Framework : GitHub - Seneral/Node_Editor_Framework: A flexible and modular Node Editor Framework for creating node based displays and editors in Unity
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Working WebGL example but it is no longer maintained and as small tweak for new Unity version.
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GraphViewPlayer : GitHub - Sahasrara/GraphViewPlayer: A simplified, player implementation of Unity's GraphViewEditor.
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More recent work but has no working example (NewGraph used a modified version of it but it’s only working in Editor at the moment GitHub - Gentlymad-Studios/NewGraph: A general node graph solution centered on data management. This is based on the idea to visualize complex data structures as graph networks without having to modify already established data classes.)
Related forum post: Simple node editor