How to disable the now default "ambient noise reduction" in macOS Catalina? In macOS Catalina, ambient noise reduction is applied by default to audio of an external source coming in through the headphone jack. This is not always desired, e.g. when recording musical instruments. In previous versions of macOS, noise reduction could be disabled via System Preferences. However, this option is not present in Catalina. This thread proposes downgrading to Mojave as a solution to the problem (which it is), though this is not desirable for me as I need Catalina for other reasons.
As mentioned in the thread, Apple's audio documentation specifies that ambient noise reduction is disabled if a 4-channel built-in microphone format is selected in Audio MIDI Setup. Unfortunately, my external audio source only supports 2 channels, so this option is unavailable in Audio MIDI Setup.
My guess is that the solution is any of the following:

Some terminal command to disable ambient noise reduction entirely.
Modifying some plist, e.g. in /Library/Preferences/Audio/.

However, I have not yet managed to find a solution, and would greatly appreciate it if anyone could think along. Thanks.
Sometimes the best solution... is a little patience! I have recently updated to MacOS Big Sur (11.0.1), and the checkbox to disable ambient noise reduction is available again in the sound input settings.
However, there is a caveat: the toggle is available only to a subset of devices. Please note the following, stated on Apple's website:

Capture less background noise when using your computer’s built-in
microphone: Select “Use ambient noise reduction”. This option does not
appear if a 4-channel built-in microphone format is selected in Audio
MIDI Setup or on Mac models with an Apple T2 chip.

Source: https://support.apple.com/en-ie/guide/mac-help/mchlp2567/mac