etymonline doesn't note that nowadays ever had a spelling with hyphen but I found a few random sites claiming that it once was hyphenated. Was it ever spelled as "now-a-days"?
Learn English – Was “nowadays” ever spelled with hyphens
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Best Answer
Google Ngram has an answer for you here The hyphen-free form is significantly more common than the hyphenated form. Google Ngram plots the frequency of words and phrases as a function of time. As you can see from the graph, nowadays is significantly more common than now-a-days".