It has been typical of all kinds of group-coverage health insurance policies to have open and closed enrollment periods. It was like that even back in the 80s and 90s when I worked. Here, the "group" is those who apply through the exchange.
I haven't read much about and don't understand why this is affecting out-of-exchange private individual policies, however.
I guess they are selling only ACA-compliant qualified plans only now. Ain't that the pits.
Anyway, a pdf of eligibility events that let you purchase a plan outside of the open enrollment window:
But these enrollment periods are split across 4 quarters. My wife’s policy ends 6/30 each year. The enrollment options are from 4/1 until near 6/30. So a smart management companies split them across four quarters for exactly staffing reasons.