I am observing a pattern on my website where pages with zero to very less traffic for about 6 months are being de-indexed. I am not sure if this is just a coincidence.
Google doesn't look directly at traffic, but they do calculate your page's importance to determine whether it should be indexed. If Google determines that your page is of very low importance on the internet, they will de-index the page. To fix this, increase your page's authority.
Web pages with less or no activity are not de-indexed by googlebot. What you are actually experiencing is google search engine cache time for those web pages that are set to expire, no follow, no cache, ect. Google follows these rule for the entire website and individual pages. https://support.google.com/websearch/answer/1687222?hl=en