How to disable the now default "ambient noise reduction" in macOS Catalina?
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