Denying berserkers shields would not make them more historical--like, check this IX century poem:
" I'll ask of the berserks, you tasters of blood,
Those intrepid heroes, how are they treated,
Those who wade out into battle?
Wolf-skinned they are called. In battle
They bear bloody shields.
Red with blood are their spears when they come to fight.
They form a closed group.
The prince in his wisdom puts trust in such men
Who hack through enemy shields."
There are also other non poetic examples of berserkers using shields, only now in modern culture it became something of a trope that they were not using shields and only hacked enemies with great axes.
As of game balance they usually have to kill someone to get shield, unless teammate sacrifice 1 eqipement slot and get it for them, I don't think it's very urgent matter to rebalance berserks, there are worse problems right now(archery i. e.)