It was custom to build altars against the wall before Vatican II, but there was not a rule. In the 1962 edition (last edition before Vatican II) of the Roman Missal, in fact, there was a diagram showing the proper incensing technique for both against-the-wall altars and freestanding altars as well. Mass can be celebrated ad orientem (with priest facing the same direction as the people, leading them in prayer) on both kinds of altars.
The Ordinary Form can, in fact, be celebrated on either type of altar. It can also be celebrated ad orientem, just as the Extraordinary Form almost always is. Even the way the GIRM is written, it is more of a suggestion that the altar be built in such a manner as to be able to walk around it. It doesn’t require it, rather it says that a freestanding altar is “desirable whenever possible.” Might I add, as well, that the GIRM does not have a preference for ad orientem celebration or versus populum celebration.
You’re right, Vatican II never mandated the destruction of high altars. In fact, it never even suggested it or even mentioned it. To be sure, Vatican II never mentioned many of the things we have today. My church, for example, an early 1900s Neo-Gothic type, has a majestic high altar with soaring reredos, which houses the veiled tabernacle, and is dressed with altar linens. In front of it, there is a smaller altar, also dressed with altar linens, that is (unfortunately) used for celebration. I am glad, though, that even though this altar is used, that the high altar was not surgically ripped out. In fact, both altars are going to be moved whenever our new church is built.
