I tested quite a few well known vst compressors recently and I couldn’t find the the SC option in some of them. Isn’t it a common option? I use Ableton.

 

if you want a bit of control over the sidechain I'd recommend Pro-C 2. Not every compressor will provide you with a SC parameter and when they do it's usually a super simple and limited control (like a HPF).

 

Technically speaking all of them have one, but not all of them allow you to do anything with it or use an external one.

 

For example Oxford Dynamic EQ only provides external sidechain in the VST3 version of the plugin. Has to do with the shit with Steinberg, phasing out VST2 and going forward with VST3.

If your compressors don't have external sidechain in VST3, then is simply not there. It can happen with many bus comps, and even emulations like 1176 and LA

 

Nope. Even when they do have an external sidechain input, its not certain that you can use it. It depends on your DAW. Some DAW developers nerf their cheaper DAW versions by not allowing sidechain routing (amongst many other things).