Can adamantine or mithral armors have magical properties (+1, for example)?

Question

As an ex-player of D&D 3.5, I'm a little bit confused about Adamantine armors and Mithral armors. In 3.5e, they are not inherently magical; it is just classic armor with a different material, so they can have magical properties.

However, in 5e, it seems they are considered magic items in some ways, but not in others; see the accepted answer to "Are weapons and armor made of Mithral or Adamantine considered magical?" for an example.

Can adamantine or mithral armors have magical properties (+1, for example)?

Answer 1

A DM or adventure writer could make such an item

D&D 5e doen't give general rules for magic item creation, iteration, crafting, or improvement. (The given crafting rules still rely heavily on DM fiat/opt-in). Magic items are primarily an adventure reward (their rules found under Treasure in the DMG).

But there's nothing stopping a DM or adventure writer making new items and combining the effects of two items is even a suggested way of making new items (or modfiying existing ones):

You can also modify an item by fusing it with properties from another item. For example, you could combine the effects of a helm of comprehending languages with those of a helm of telepathy into a single helmet. This makes the item more powerful (and probably increases its rarity), but it won't break your game.

— Dungeon Master's Guide, Creating a Magic Item, Modifiying an Item, p. 284