Yes, Many Glacier is open in 2021. However, there are a couple of things you need to consider. First, Many Glacier Road is closed due to construction and is slated to reopen in 2022. For this reason, you may be unable to reach the Many Glacier area. Furthermore, most of the lodgings, facilities, and trails begin to shut down after September 14, 2021. For Red Bus tours and other park facilities, wearing a mask is required, regardless of your vaccination status. You can check for updates on the official website.