As expected, I have received my usual alerts from BBC news.
In the new notifications section in Settings, all the apps I’ve allowed notifications from are switched on. There is a slide switch to turn them on or off. All your apps are listed in the notification settings
In the Libre App in the Settings, all alerts are switched on, but in the Libre section there is a Critical Alert part. That too is ticked. It takes only a moment to switch on or off, not all the messing about as described in the OP, but if it was allowed in your older version of iOS it will already be switched on.
So as I said above, you will almost certainly have to do nothing after the upgrade to allow the Libre App to work. Just check in the notifications section in Settings and click on the Libre App. There you will see all the alerts switched on, including critical alerts. In the
unlikely event they aren’t, just turn them on. A whole four taps of the finger. Big deal.
This has all been done by Apple to make life easier. Sometimes alerts get tedious, or maybe you want to switch them off for a bit peace and quiet. It was tricky to do before. Now you just go into Settings, then notifications, click on the Apps and use the slider at the top of the screen to turn them off. It couldn’t be simpler.
It may be, of course, that Android phones have a similar easy system, but I suspect they don’t.
By the way, I find I have a new Notifications section on my watch (Apple, of course) which operates when the phone is not actively in use, so Libre alerts can (and do) come through on the watch. This worked before. When I was sitting at the Rovers, when they scored, seconds later my watch informed me of the score.