As others have said, you'll welcome to have backpacks with you in the line, but you'll have to take them with you on the ride. Most times you can just shove them between your legs on the floor and hold them in place. Most of the Disney coasters are pretty tame so you generally won't have a problem holding onto the pack. I've done this on Big Thunder, Matterhorn, and Space.
It's a little more awkward on Star Tours and Indy because there isn't a lot of space on the floor. For Indy, avoid being seated in the wheel seat (front row) because the steering wheel gets in the way and makes it hard to wedge things in/out.
If you're a large or tall or long-legged person, you might have some problems on Matterhorn. The cars can be pretty cramped to start with, so getting a backpack wedge in front of you, might be uncomfortable.
The only ride where you can leave things at the "station" is Soarin. If you can't fit your bag in the under the seat compartment, you can leave it on the floor. The CMs were directing guests to do that last week, especially with packages and other bulky items. I should add that for this ride, you cannot hold items with you. They either must fit in the seat compartment or be left on the floor. They don't want folks to accidentally drop something and hit one of the riders below.