Question about Lowepro Whistler BP 450 AW

Yes you can. As a frequent flyer who often flies within the USA, I can tell you that this is not a problem. Many LCCs (low-cost airlines) such as Spirit or Jetblue have a kind of template at the gate where passengers must be able to check in their hand luggage. If it doesn't fit, it has to be checked in at a high cost.
However, as the previous speaker has already pointed out, the maximum dimensions are specified, but with a backpack this is not a problem due to the flexibility of the fabric. It becomes a problem with a hard-shell case.

If I take Swiss as a guideline, then no. With Swiss, hand luggage may have the following dimensions: 55 cm high, 40 cm wide, 23 cm deep. But since the backpack is flexible, I think it could still go through if it's not packed to the brim.

But since you don't write which airline you want to fly with, it would be a good idea to check directly with the airline's website. There are not only differences between the airlines (easyjet 56x45x25 cm, Ryan Air 55x40x20 cm, Alitalia 55x35x25 cm), but also depending on the destination. Low-cost airlines tend to be less flexible.