I can only add my experience from IF, not prolonged fasting. When i was regularly 18:6 my periods were lighter with less cramping. Way more manageable than they were before. I know some people don’t have good experiences or they lose their period, but for me it was a change in the right direction. Got pregnant first try last year, so it didn’t impact fertility for me.
I get reduced clotting and discomfort and lighter flow, for me it’s an excellent treatment for painful and disruptive cycles. However, the way I found out I had very low iron was when I almost fainted while fasting on my cycle, it took a few months to clear up before I could fast on my cycle again. Now I try to fast before my cycle but it doesn’t always align right.