Sidechaining is the technique of gating one channel with the envelope of another - doesn't have to be applied to bass guitar+kick drum - its also popular as a technique in synthesis, such as on a 303 acid line, gated by whichever part of the rythmn track is most relevant to the cause, or in eurorack modular context when one module is gated by another through a side chain signal. It is also highly effective when applied to vocals too.
I agree, there are countless both purely engineering and creative use cases for sidechaining!
My absolutely adored kind of sidechaining is spectral, that is, when it's not merely a loudness envelope of a source that drives the gain of a target, but when both are split into FFT bins and the envelope of each bin of the source drives the gain of the corresponding bin in the target.
That allows for carving out the target signal with the frequency response of the source, surgically. Works miracles is modern bass-heavy styles, along with spectral limiting.
There is one particularly amazing VST plugin at this, but I won't advertise here.