They are envelope shapers or "enveloper" plugins for situations where you do not want to use a compressor sidechained, with the effect applied when there is no material there for the sidechain which the compressor can react to. But that is also why you will see people create a midi track with data (any instrument muted), and then sidechain to that. There doesn't need to be any sidechained audio signal for the compressor to react to.

There are tons of them: https://www.pluginboutique.com/categories/63-Envelope-Shaper

Curtiss King Midi Duck has one of my favorite plugin interfaces ever. Who comes up with this stuff?

 

FX23 Psyphaz Pro is the best one I've found so far. Devious machines Duck is more basic and probably a bit faster to work with for most people, but Psyphaz has everything you need to perfectly phase align kicks with bass, EQ, compression, waveshaping and all the analyzers you need for subs and bass.

https://fx23.net/PSYPHAZ-PRO/

NOTE: Win only