I can either use the apple account on the latest iOS or on the old Mac mini but not on both. Basically when I’ve tried to log in on the iPhone it told me I need to upgrade the Mac mini or remove it if I want to use the account on my iPhone. Obviously I removed the account from Mac mini. Perhaps I can create new account specifically for Mac mini but what’s the point to have different iCloud iMac accounts for each device? I’m confident but at some point this trick will stop working as well.