I have asked before for people to explain when they have used the term "linguistic register" since I had no idea what is. I had received no replies to date.
For myself and others, I looked up a definition: " "Linguistic register” refers to the concept of adapting one’s use of language to conform to standards or traditions in a given professional or social situation". All it means is adapt your language to the situation in which you find yourself.
So, lemmingofdestiny is saying "henceforth" is a word typically used in a professional or technical communication rather than in casual conversation.