The docs say "Called when a player is attempting to spawn for the first time".  I interpreted that as "first time since player data was wiped".  But a player who played just yesterday triggered this on his first spawn, then again today.  I have not wiped player data.  What exactly does "first time" mean in this case?

That is when the player spawns on the server after connection to it.

Hmm...  if I want to get the very first time a player connects after a wipe, what should I check?  I think I remember seeing a "past lives" thing somewhere...

There isn't a hook for that. You'd have to either store and check something, check for a permission, or check if Rust has something that can provide that.

Does this look correct for what I'm trying to do?

object OnPlayerSpawn(BasePlayer player)
{
       if (!player.hasPreviousLife && player.lifeStory.metersWalked == 0)
       {
              //first time player ever spawned in the server since last player data wipe

That may work.