This is a concern many folks seem to have. But it makes little sense. Some of us drivers of mature years learnt on a straight gearbox and, over the years, were presented with all manner of other gearboxs. No instruction was offered and we simply got on with it.
There are those who will seek out a trainer using a manual. Which is great, except that you’ll only know how to use that particular type of gearbox and that wont be a lot of help when presented with something completely different. Also, bear in mind that such training vehicles tend to be quite elderly. So you’re in a position where you choose to learn on a truck that’s probably been to the moon and back simply for the privilege of making life harder to pass the test.
Also, bear in mind that the huge majority of trucks under 10 years old are auto. Yes, there are manuals about. But, up to 18 tonnes, any manual is likely to be a 6 speed (conventional) box.
So, all in all, my advice is not to worry about that. In addition, you have the advantage of utube which has demos of every conceivable gearbox.
Unless we’re suggesting that the modern driver is mentally incapable, I honestly see no problem in passing on auto.
All the best, Pete 