I'd guess that if you consistently avoid a left turn it could be because it's forbidden instead of because you miss it every single time. It's safe to assume that by now they take this kind of implicit feedback, but it either takes a long time to be processed or it's not happening on time-restricted left turns..
To highlight this, it is literally impossible to choose your own route. Google will randomly redirect you, will keep nagging, while you constantly have to click 'no'.
For example, let's say you are driving from New York to California. There are two main paths, dictated by the mountains.
Google knows this, and lets you pick the southern, or northern route.
The southern route is 5 hours longer, but gee, no snow! Google doesn't have "avoid snow", and 5 hours longer becomes much faster, if the northern route has a storm.
So you pick the southern route.
Congrats! For the next 50 hours of driving, google will happily exclaim "I found a faster route!" and switch you over, unless you click cancel.
This means that you are constantly, sometimes as often as every 5 minutes, forced to interact with maps while driving, or be forced to redirect as google wishes.
This 5 year old bug is well known, people complain about it everywhere, but google could care less.
The worst thing is for maps to be "silent" when there's a route available that can save you half a day of driving. You don't want that but an average driver does.
You are basically a power user who knows better than the map which is fine, but then just add a waypoint that forces the Southern route and go on with your life.
For the average user, nagging them to get the 5 hour faster route is the right strategy.
Extremes much?
You act as if there are two options only. Relentlessly nag the user, and not tell them at all.
There is another. Ask the user, and be done with it.
When I click on decline, I mean it. Everyone does. And it isn't just nagging, it literally forces you to click no every time, or your route changes.
When in traffic. In seconds. Immediately. Over and over.
It is a terrible design choice.
I wouldn't count on that. I've been exiting a Costco for years now where Google Maps tells you to take an impossible left turn due to a divided road. Google should have a mountain of data showing those directions are never, ever, taken. Street view shows it to be impossible too.
If they are actually looking at that type of data they aren't doing much with it.