Couple of questions.
I've never come across a "Side Chain Limiter". What would someone use one for? How would it work?
Secondly, the Ca-2a is definitely not a limiter. It has a "Limit" mode, but I guess this is to echo the limit mode on the emulated hardware. It does not work as the modern understanding of a limiter would.
Thirdly, what is wrong with the Sonitus? It may not be VST, but neither is an original Teletronix La2a or a Pultec EQP-1A Program Equalizer. If they work well, why does it matter if they're "Legacy"?
My main point is still, why/how would one use a sidechain limiter? Please excuse my ignorance.