Those ‘?’ counters were called Concealment counters, and are still in use today in Advanced Squad Leader, the direct successor to Squad Leader. There are a number of other methods used to create a fog of war effect in todays wargames. You have blocks which represent your side’s forces, which are visible only to you - Columbia Games has quite a few of those titles.
There are card driven games, starting with We the People - which simulated the Revolutionary War in which you are dealt a hand of cards each turn, and the cards are used to move your counters on the map. The fog of war comes in because you never know what cards your opponent has or what capabilities those cards give him/her.
The ‘untried units’ was a core feature of an SPI game called Panzergruppe Guderian, and has been carried over into a number of different games as well.
One of the most interesting mechanics I’ve played is an old Avalon Hill game called Flattop, which covered the carrier battles in the Solomon Islands, in which you moved your units on a paper map, and only placed them on the big map once they were spotted. Incredibly tense, as you were often worried that you were about to get crushed by an unseen enemy airstrike. Also, I believe there was a similiar mechanic in Midway by AH as well.
So, yes, there are lots of ways to introduce a fog of war aspect into a game.