Structure related mobs can not spawn in superflat worlds.
This is true for:
Pillager
Witches
Guardians
Steps taken to confirm this:
Forward Confirmation
Generated World on any Seed with Superflat (flat) World Generation and included all available structures (default spawn ratio)
Using admist and /locate to find nearest structure, teleport to it, and confirm no spawning
Save and exit.
Using NBTExplorer change world generation type from 'flat' to 'default'
Join world and confirm spawning entities
Confirm changed world-gen by loading new chunks
Backward Confirmation
Generated new default World on any Seed
Using /locate to find and teleport to nearest structure. Confirm through admist as well.
Save and exit.
Using NBTExplorer change world generation type from 'default' to 'flat'
Join world and kill all Entities (except player) to confirm no spawning entities.
Confirm changed world-gen by loading new chunks
In addition, seed-changes were tested, as well as importing confirmed-working chunks into superflat world and manually creating structures through NBTExplorer.
In all these cases structure entities consistently did not spawn while level.dat contained 'flat' - type world generation.
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You've convinced me, confirmed 🙂
Poking in the code it seems that yes there is a witch spawned by the world generator by default when the hut is initially created, but after that, only the regular mobs would spawn in there. I tried deconstructing the hut and rebuilding the floor at different heights, but nothing would force another witch to spawn there.
Removed "CLONE" in the title.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Tested in 1.5 and the bug is still there!
Still a bug in 1.7.2
The strange part is that the "area" in Temple.dat under the data folder is generated and looks to be correct.
But the flatland code seems to ignore that data.

Is this still a concern in the current Minecraft version 1.7.4 / Launcher version 1.3.8 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Can confirm, still a problem in release 1.7.9.
I also was able to confirm that this is directly a result of the world generation settings. I generated a flatland using the settings in the description above, normal mobs spawned (see image generator_flat.png).
I then used NBTExplorer and edited the "generatorName" value in level.dat from "flat" to "default". After reloading the world, only witches spawned (see generator_default.png).

Is this still a concern in the current Minecraft version 1.8.1 Prerelease 3 / Launcher version 1.5.3 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Yes, confirmed that it is still a problem for 1.8.1 Prerelease 3
Above directions still apply, use the following for superflat preset:
3;2*minecraft:bedrock,60*minecraft:dirt,minecraft:stone_slab;6;biome_1
Seems similar in many ways to MC-59314 (Guardians not spawning on superflat worlds).
Different details, also much easier to test (just create a superflat world using preset "Water World").
Still an issue in 1.8.2-pre7
Still an issue in 1.8.4
Still an issue in 1.8.8 and the newest 1.9 snapshot 15w45a.

Can confirm that this issue is still present in all versions after the OP's reported versions. For creators trying to make a superflat void world for their Skyblock maps only regular mobs will spawn and Guardian Temples will not even spawn in.
Testing regarding this issue, this issue is superseded by issue MC-169728
Edit: No longer the case, as moderator merged posts.
Confirmed for 1.15.2 and 20w12a
Can confirm, this is still an issue for 1.16 pre-2. I spent a few hours pouring over nbt data trying to figure out why my outpost wasn't working and weeded it out to world type. Also, I tried a superflat preset explicitely telling the game to generate the outposts to find out it didn't work anyway.
In 1.16.1 and 20w28a
Can confirm for 21w20a
Still the case in 1.18.1
The witch pressent in the pic "Can't reproduce in 1.4.5.jpg" is generated with the hut and not spawned in in the hut.
In the screenshot i posted a skeleton has spawned in the hut, and that only happens in flat worlds.
In normal worlds the only mob that can spawn in a hut is the witch!
So i still claim this to be a bug.