The ordinary definition of "unusual" refers to frequency in the current environment; there is nothing inconsistent with textualism to understanding that the use of that word in the 8th Amendment very much meant exactly that the frequency of practice of particular punishments in some context (whether global, or among "civilized nations", or whatever) would be relevant in determining whether or not it was "unusual".

Its not a changing definition when the ordinary definition of a word refers to circumstances which are subject to change.