You can't possibly draw one single rule. In this game there is no consistency anymore. That's the problem, imo.
My level 1 char can use the insane title, my level 20 noobie char rides Invincible.
My level 90 char in mop couldn't ride the serpents that my level 90 shaman main has unlocked from serpent riders (it has been changed when the content became obsolete). But still was able to ride the shado pan tigers. My alt can't use a stupid Draenor toy from the tanaan factions, but she can use various profession related items she has no idea about (archaeology items), and old reputation items that became toys with the toybox.
I bought pets from factions when mop added pet battles. All my character can pull out any pets.
I can use Salyin Battle banner anywhere. In Legion it's restricted to Pandaria, among many other cool items.
Now, try to draw ONE rule from this mess please.
My observation on this: old items that are no relevant anymore (but subject to somekind of revamp like pets or toybox) usually get added into the shared pot retrospectively. Current items have restrictions. Now. Old quest rewards fall into the obsolete category imo, and thus, should be added into the shared pot.