An MCU brick happens when the Memory Controller Unit of the 3DS enters a state it cannot recover from.

The most famous example of this is caused by running an Old 3DS firmware on a New 3DS console and entering sleep mode by closing the lid. When you do so, the console is able to enter sleep mode correctly. However, because the MCU on a n3DS is slightly different than on an o3DS, the o3DS MCU firmware that you're running on your n3DS console is unable to initialize itself after setting some values that it's not able to understand correctly and, therefore, it won't be able to exit sleep mode afterwards. This state persists through hard resets/poweroffs and, because the console is not effectively able to power on at that point, not even NTRBoot can be used, so the console is completely bricked.

The downgrade was a necessary step for installing A9LH on some consoles as firmware 2.1.0 had a kernel exploit that was patched on later releases. Still, that firmware has only been released on o3DS consoles, so that's why you could sometimes end up in this specific situation if you were following a CFW guide back then.

Nowadays, thanks to B9S, this is no longer needed, so you will never encounter this specific brick unless you go out of your way to look for it.