Learn English – the difference between “universal” and “generic”

Something illegal is forbidden by law: "It is illegal for motocyclists [sic] not to wear a crash helmet."

If a thing is illicit it is done by someone who knows that it is disallowed by law but that under different circumstances it could be legal: "The crew were involved in the illicit import of brandy"

(it is basically legal to import brandy but not the way they did it).

This is backed up by things like illicit sexual relationship. It's not wrong to have a sexual relationship normally, but it could be "wrong" if you are married, or not yet married, etc. illicit trade is another such example. It's not wrong to trade, unless you do it in the wrong way.

Supposably does mean "it is possible to suppose", as opposed to the very different meaning of "supposedly"!

Supposably is quite rare in actual use; I believe it's more an inkhorn term than one actually found in the wild, so to speak. Thus, I fear that when you encounter it you are probably really encountering somebody who meant to say "supposedly".