It doesn’t seem like this is possible... but yet the Adamantine property indicates that it can be applied to any armor. Is this on purpose? Will it change at some point?
Ah, I misunderstood what you meant. Seems like maybe the D&D Beyond folks have to manually create each entry for the specific adamantine armor types and haven't done so for the spiked armor since it's a special armor type that was added later.
It doesn’t seem like this is possible... but yet the Adamantine property indicates that it can be applied to any armor. Is this on purpose? Will it change at some point?
Why shouldn't it be possible?
Well It is possible, but not showing on DnD beyond as allowable.
Ah, I misunderstood what you meant. Seems like maybe the D&D Beyond folks have to manually create each entry for the specific adamantine armor types and haven't done so for the spiked armor since it's a special armor type that was added later.
So, an oversight on the part of the staff.
Spiked armor is a special kind of armor. It isn't in any books' equipment list, it only exists in the battlerager's feature description.
Stuff like that is kind of vague rules wise, and it doesn't get treated as "any" of its item type, especially when it isn't in the core rulebooks.
A DM could rule otherwise and homebrewing magic spiked armor is simple enough.