I'll confess I'm kind of sad the question about checking for default/unset vector and quaternion values got deleted by the asker. I had half an answer written going into the gotchas of equals operators on vectors and the difference between quaternion identity and default. 😅

@DMGregory That's partially on me then. I read it as more of a basic programming confusion (identity being equated to null), answered in comments & posted a close vote. Your nuanced answer that you teed up tell me I was hasty & didn't appreciate the potential depth.

Is it possible for a mod to ping the user & suggest an undelete?

If it matters, it looks like they reposted it w/ some tweaks over on SO (link).

Looks like they've got an answer using nullable structs, which is the main thing I was going to recommend anyway, so they're probably covered.

I agree with you, "how do I check if a struct is unassigned" is general programming. But as we get into the particulars of loading save files in Unity, using Unity-specific types, there are potential gamedev-specific inflections.

Kind of like that Q&A we have about how to troubleshoot null reference exceptions in Unity. The error class is generic, but in that engine it shows up in particular ways that a game developer could explain in better detail than other C# programmers.