I also miss that functionality at VST versions..
Just one thing – additional input channels (ones used for ext. sidechain) are definitely possible to implement also for VST 2.4, it isn’t exclusive feature of VST 3. Sidechained stereo plugins just appears as 4in/2out.
There are quite few existing plugins, which uses that and works pretty well across multiple VST hosts. For example – NI Vintage comps, Softube comps, free ReaComp from Cockos’s ReaPlugs http://www.reaper.fm/reaplugs/ etc.
As for VST3, I don’t think it has some practical advantages besides Steinberg’s world particularly for audio effects.. Maybe there is possible sample accurate automation (eg. plugin can get automation values update in between of DAW processing block boundaries), but I’m not sure if that is so significant.
I have AnthologyX and I really miss VST2.4 support for sidechain on your plugins. A real shame because, as Michal says, this is quite a common feature now for plugins. I never use VST3 unless I really need to, with Reaper support for VST3 being, from what I have read, rather flakey in the past. It does seem, from what I have read, that Reaper now handles VST3 much better but plain old VST2.4 sidechain support (“works pretty well across multiple VST hosts”) would definately be my preferred method of external sidechaining.