According to the following Google Ngram, in the U.K. the modals should, shall, and must were virtually missing from English writing during the 18th Century (I've added will for a comparison modal which was unaffected).
I have never seen this mentioned anywhere, and I couldn't find it in a brief web search. What happened? Was this a real phenomenon, or could this somehow be an artifact of Google Ngrams? Were these modals absent from speech, or just formal writing? More interestingly, how did they get reintroduced? A brief search shows that Shakespeare definitely used these words quite frequently.