In the Barrows in MM7, which form kind of a complicated teleporter maze, most of the panels showing teleporter destinations start locked. According to the Prima guide, there were supposed to be three keys, one in each of the Barrows accessible from the outside (specifically Barrows VII, IX, and X) that would unlock all the panels. However, from what I've read (and what I recall when I last played the game years ago) is that the key in Barrow VII was not included. With only two keys, some of the panels cannot be unlocked, making navigating in the Barrows much harder (it's hard enough even when you know all the teleporter destinations).

Was the missing key a deliberate decision on the designers' part or a bug? If a bug, has anyone ever patched the game to restore the missing key? (I'm pretty sure NWC never did so.)

Two I remember:
High Ledge Chest
Library Chest

Trying to remember the third.

I play with the greyface patch and this never happened to me. Everything is unlocked and visible - and I always get all three keys -
eventually. I do a lot of back and forth.
To help me, I write down which barrow i've visited, every game, and open every chest and find every secret.
It takes quite a while before every panel is visible though, you need to be almost finished.

I was hoping the Greyface patch fixed the key problem, but I couldn't find it mentioned in the patch's list of changes. If I ever play the game again, I'll make sure to use the patch.

I just use the basic GOG install, no Greyface, and I'm still able to open all the closed hatches eventually. I just checked an old save, and yes, all barrow panels (1-15) are open.

Keep searching the barrows, the keys are in there somewhere. I don't recall where. Make sure to check behind all the various secret doors and panels and so on.

Yes, I believe they're supposed to be hidden in various places in Barrows VII, IX, and X. These are the three Barrows that are accessible from outside on the Barrow Downs (a restatement of my original post).

Admittedly, I was using Mike Marcelis's mostly very good walkthrough on the-spoiler.com. Mostly very good, that is, but still incomplete in a number a ways. (This seems to be one of them). (His MM6 guide on the same website is much more complete and thorough. I think he may also have written the MM7 guide for an early unpatched version of the game and never updated it.)

I honestly couldn't tell you which burrow had each key, or even recall how many keys there are. I just know all the panels can be opened, which means the keys are in there somewhere. :)

Well, as I mentioned in my original post, the Prima guide asserts that they are in Barrows VII, IX, and X.

While it's true many Prima guides are riddled with errors and only marginally useful, I've found the MM7 guide (and to a fair extent the MM6 guide, though not as much as Mike Marcelais's guide on the-spoiler.com) fairly helpful in planning out how to play the game. (It does have some errors, of course.)

If you find a chest that is highly charged but empty, then re-open it. See if the key appears. Because the keys are always in the most dangerous chests. Sometimes things appear on the second opening but not the first.

It is included, but the devs were sneaky. First find a button to open the secret area. Inside is a chest, easily UNDERlooked (read macAilpin's reply). To reach it, find an elevator. VII is the nearest entrance to Stone City. If we start exploring the lower numbers first, we go to IX, V and so on; this will make X, the other one with a locked panel, one of the last we get to. [EDIT: but it doesn't have to be, we can go straight to it from VII]

The Flamestryke page has the solution to finding the keys.

https://www.zimlab.com/wizardry/recovered/flamestryke/mm7/flamestrykes_mm7.html