I am having difficulty understanding this. You are upset that Paradox removed a cheap tactic? (At least, in my opinion, it sounds pretty cheap. But to survive with Byzantium I guess you got to cheat a little bit.)
The current situation appears very much like a bug. It has impacted a Byzantium strategy, but I cannot believe that was intentional. Or if it was, the implementation is poor enough to be regarded as a bug.
When AnyNation vassalises a nation who was at war, AnyNation will inherit that war. AnyNation can then not offer either Return Core, Cancel Vassal or Release Nation as peace treaty items. Basically, half of its peace treaty options have gone.
So if AnyNation vassalises a nation and then inherits a war it didn't expect, or inherits a war it thinks it can win but then finds it cannot, it is in huge trouble; it can't use half of the standard peace treaty options to appease their enemies, and will probably lose half its territory.
This has to be a bug. If it was intentional, surely they would only disable Cancel Vassal at most - and even that would be a very poor implementation; the only method that would make any real sense is if you weren't allowed to Cancel the vassal you just made. That would serve to limit the Byzantium strategy, without also breaking many other situations which have nothing to do with this.
So yeah, I think it's just a bug, and I have raised it as such: [h=1]
[1.5.0] Inheriting war does not allow Offer Tribute Cancel Vassal or Release Nation[/h]