How often are End Portal rooms inaccessible?

Firstly, Eyes of Ender don't always necessarily lead to an End Portal. They instead locate the nearest Stronghold (emphasis mine):

Pressing use while holding an eye of ender will cause it to fly approximately 12 meters in the direction of the nearest stronghold, traveling through any blocks necessary, and leave a trail of purple particles in its wake, the same particle effect used for ender chests.

Since Stronghold generation is random, to the point where sometimes an End Portal room is not generated (in some versions this causes strongholds to be re-generated until and End Portal is placed; in others only one Stronghold will contain an End Portal^{[citation needed]}), this can also lead to rooms being blocked off naturally:

Sometimes, doors can be sealed off by stone bricks, resulting in "secret doors", its usually occur on 5-way crossing.

So in the instance of locating a "blocked off" End portal, your best bet is simply to go mining. If a wall does not have a door, break the blocks behind them to see if another layer of stone bricks is visible. This should indicate a "hidden" room. You won't need to dig more than 2 layers at most. Though, in my experience of playing the Java Edition, using an Eye of Ender above ground, to the point where it locates a Stronghold has generally led me directly to the End Portal (though I have only played intermittently since the release of 1.14).