Your question pertains to an arguable point. I tried to frame the options in such a way as to allow for both an absolute moral imperative as well as a relative moral claim that would be contingent on context.
You seem to be arguing that the acceptability is dependent on historical context. In that case you might have chosen "It use to be acceptable, but it is no longer acceptable to execute heretics." Or you might have chosen, "It is sometimes acceptable to execute heretics" and then argued the "sometimes" is dependent on the historical context.
I stated in the OP my position which is: If it is wrong today, then it was wrong then. I put it that way in order to convey that it is always wrong to execute heretics. That is an absolute. Therefore, I chose "It is never acceptable to execute heretics." I see why you might be reading that in the present tense. The "is never" is intended to convey an absolute. Does that help?
If you take the position that the moral imperative is an absolute, as I do, then even though they found it acceptable in the past doesn't somehow make it right. On the contrary, if it is never acceptable, then they were wrong. An absolute moral imperative doesn't change with context, presumably.
Again, all of this arguable. So, I look forward to your argument that, in this instance, the acceptability of executing heretics depends on the context.