Royal Skousen discusses the use of "armshield" in the Book of Mormon and related terms in Early Modern English.
Royal Skousen, The History of the Text of the Book of Mormon: Part Three. The Nature of the Original Language (Provo, UT: Foundation for Ancient Research and Mormon Studies, 2018), 449–451
Armshields
In describing the armor in the Book of Mormon, the text refers to armshields and headplates. Although neither word is listed in the Oxford English Dictionary (at least not with their Book of Mormon meanings as types of armor), they are clearly acceptable compounds in English, and in fact necessary for understanding the text’s descriptions of armor. The word armshield occurs twice in the text and obviously refers to armor protecting the arms:
Alma 43:19:
when the armies of the Lamanites saw that the people of Nephi—
or that Moroni had prepared his people with breastplates and with armshields
yea and also shields to defend their heads—
and also they were dressed which thick clothing
Alma 43:38
while on the other hand there was now and then
a man fell among the Nephites by their wounds and the loss of blood
they being shielded from the more vital parts of the body—
or the more vital parts of the body being shielded from the strokes of the Lamanites
by their breastplates and their armshields and their headplates
The first of these verses show that shields alone can also be used to refer to “head shields”, but the second verse shows that there is an actual term in the text for “head shields”, namely headplates. Elsewhere the word shields seems to indicate that various kinds of “arm shields”, at least, are being referred to, especially in lists where the term headplate is also conjoined:
Alma 46:13
and he fastened on his headplate and his breastplate and his shields
and girded on his armor about his loins
Alma 49:24
but they were shielded by their shields and their breastplates and their headplates
insomuch that their wounds were upon their legs
many of which were very severe
Helaman 1:14
the Lamanites had gathered together an innumerable army of men
and armed them with swords and with scimitars and with bows and with arrows
and with headplates and with breastplates and with all manner of shields of every kind
Ether 15:15
both men women and children being armed with weapons of war
having shields and breastplates and headplates
and being clothed after the manner of war
The example of Alma 49:24 strongly implies that the Nephites had no “leg shields” (or “leg plates”); thus the reference to shields in that passage is probably restricted to “arm shields”. But in other cases, shields seems to be simply referring to any kind of shield, which could include headplates.
Aa 43:21
but they were not armed with breastplates nor shields
Alma 44:9
behold it is your breastplates and your shields that hath preserved you
Alma 49:6
yea and they had also prepared themselves with shields and with breastplates
and they had also prepared themselves with garments of skins
yea very thick garments to cover their nakedness
3 Nephi 3:26
and Gidgiddoni did cause that they should make weapons of ear of every kind
that they should be strong with armor and with shields and with bucklers
after the manner of his instructions
Ether 15:24
and they contended in their mights
with their swords and with their shields all that day
The list in 3 Nephi 3:26 includes bucklers, which (according to the OED) means a small round shield. Under definition 1, the OED adds: “but sometimes it was larger, and fastened by straps to the arm”—in other words, another kind of “arm shield.”
Although I have not been able to find evidence for the word armshield in Early Modern English, there is evidence for such protective armor, but they were referred to as arm-pieces, as in this example found on Eighteenth Century Collections Online (ECCO) where an accompanying illustration of the soldier and his armor is enumerated in English and Latin:
1705, Charles Hoole (translator), Johann Amos Comenius’ Visible World
If we be to make war, (1) soldiers are listed [enlisted]. Their arms are (2) a head-piece, (which is adorned with a crest) and the armor, whose parts are (3) a collar, (4) a breast-plate, (5) arm-pieces, (6) leg-pieces, (7) greaves, with (8) a cost of mail and (9) a buckler.
As can be seen in the previous examples, the compound breastplate is very common in the text, and obviously the compound headplate is parallel to breastplate. The word headplate occurs one more time in the text with breastplate:
Alma 43:44
yea they did fight like dragons
and many of the Nephites were slain by their head
yea for they did smite in two many of their headplates
and they did pierce many of their breastplates
and they did smite off many of their arms
Besides the ten instances of breastplate(s) already cited, we have this addition instance:
Mosiah 8:10
and behold also they have brought breastplates which are large
and they are of brass and of copper and are perfectly sound
The King James Bible has the word breastplate (spelled without a hyphen in the current text but with variation in the original 1611 printing). As we might expect, there are no instances of armshield or headplate in the King James Bible. To be sure, breastplates is listed in the OED, under breast-plate (although some citations in the OED lack the hyphen). Historically, the Book of Mormon editions have had the spelling with the hyphen, but not in the current LDS edition (or for most instances of the word in the current RLDS edition).
Of course, speakers of English can create compounds like armshield and headplate as needed. In fact, the OED has a separate listing for the compound head-plate, but with different meanings than the one used to refer to armor in the Book of Mormon. There are four meanings, with their first citations dating from 1647, 1781, 1836, and 1855, respectively; the first two are listed as historical, rare, and obsolete. In Early Modern English, there are other creative instances of the compound headplate. Although the following two examples from Early English Books Online do not refer to armor protecting the head, they show the ability of speakers to create compounds referring to objects placed on the head. In the first citation, headplate is a priest’s mitre; and in the second one, headplate is used to describe an imperial crown:
1692, John Edwards, A Father Inquiry into Several Remarkable Texts of the Old and New Testament
The words Urim and Thummim in the breastplate answered
to those Holiness to the Lord in the head-plate or mitre.
1695, Maximilien Mission, A New Voyage to Italy
There is a cross on the top of all,
and a semicircle supported between the two head-plates behind,
which is raised above the bonnet, and joined to the top of the cross.
And there is now even an example, albeit a modern one, on the online, third edition of the OED of headplate occurring along with breastplate, and both within the same list of archaeological artefacts. Here the meaning for headplate appears to be close to how the Book of Mormon uses this compound noun:
2001, Robert B. Drake, A History of Appalachia
Hopewell burial remains within the conical mounds
have yielded copper and iron breastplates and necklaces,
antlered head-plates made of cooper, rings, beads, . . .